986 wouldn't start for over a week and now it's fine?

I have driven my 2000 Boxster almost every day the past few months, but the beginning of week before last I was going to drive it to work but it would crank but not start.  I didn't have time to mess with it and have other working vehicles so I just drove a different car. 

A few days later I tried it again and still crank but no start.  I finally had time to look at it today, and I checked the codes and didn't see anything, and I tried to start it again and it started and runs and drives fine. 

What should I do now? I'm sure it's not permanently fixed and it will probably die again in some really inconvenient place, but there is also hardly any point it having it if I can only drive it somewhere that isn't important to get to and I have time to wait 3-4 hours for a tow truck if it won't start again.

You observed it, which altered its state.

First, what does "wouldn't start" mean?

Turn the key and absolutely nothing else (no lights, no hums, no relay clicks, etc) happens?

Lights come on when you turn the key but nothing else happens?

Lights come one, starter clicks, but then nothing else happens?

Lights come on and fuel pump primes but no starter click?

Lights come on and fuel pump primes then starter clicks and nothing else happens?

Lights come on and fuel pump primes, starter clicks then the engine spins, but no combustion?

All of the above but with sputtering when the starter is spinning but the engine won't light off? 

All of the above but with a nice rev before falling flat on its face?

I did not have time to verify whether the fuel pump primed before it started working again, but everything else normal (lights, starter, etc), but no combustion at all.  It appeared to be either no fuel or no spark for some reason.  

Would it be a silly idea to just guess at what's wrong and change the fuel pump, ignition switch and fuel pump relay and see if it dies again?  I think otherwise I will probably just get rid of it, I am not interested in having it towed home a bunch of times until it stays dead long enough to find the problem.

Tk8398 said:

Would it be a silly idea to just guess at what's wrong and change the fuel pump, ignition switch and fuel pump relay and see if it dies again?  I think otherwise I will probably just get rid of it, I am not interested in having it towed home a bunch of times until it stays dead long enough to find the problem.

Replacing parts won't gain you any confidence given that you don't know which parts are an issue. I would drive it a few times in a less critical scenario and see if the issue re-presents. If it does, you've got a place to start diagnosis. If not, you're back to square one. 

The first thing I'm checking is battery cable terminals and all body grounds are tight and clean. Have your own scan tool to check if it leaves you stranded?

Just a long shot.....

if you disconnect the battery will it reset the computer ?

Maybe bring tools to disconnect if it dies again 

Also on the old Bosch injection  (914 Porsche) all the grounds needed to be clean or it would run badly ........

I'm really not sure if it's a computer issue or just a failing electrical component.  I would guess it's either something with the fuel pump (bad pump, ignition switch, relay), one of the other sensors, or an immobilizer issue. I do have a scanner that can read some data although I don't have the one that can read all of the data from the sensors.  

I will try driving it more as is and see if it has an issue again, but it will be of limited use if I only drive it when I won't mind having it towed home if needed.  

I don't really expect to last much longer either way because it's already probably in the highest 5% of how many miles Boxster have lasted, but I really don't feel like dealing with finding some to replace it with if I can avoid it a bit longer.

I hate throwing parts at cars without a positive diagnosis of failure, or even a direction to look, because it's an industry standard that New stands for Never Ever Worked.  By adding a new part you are adding a potential problem, which will make testing more difficult.

I bought a nonrunner GTI on GRM a ways back and one of the things I found was that previous attempts to correct the no-start actually caused issues.  The ignition module connector had loose terminals from multiple unplug/plug cycles, the distributor was wired wrong, the cam was off time, and some other things.  The ultimate cause was a faulty tach that was preventing the engine from getting spark, disconnect the tach and it'd run.  After fixing the other, wrench-induced errors.

In YOUR case, however, I wouldn't feel bad about replacing the fuel pump relay.  The contacts in them get poor with age, and when they start to fail they will work only sometimes, depending on how the contacts snap together on that cycle.  They're also a wearing moving item, which is more likely to fail than untouched wiring.  They can also be rather temperature sensitive, a relay that doesn't work when it is freezing outside may make better contact when it warms up and the metal inside changes position ever so slightly.  Also, new relays are rarely bad out of the box, so you won't be inducing a fault by replacing it.

If I had the car in my bay, I'd replace the relay with a loop of wire to use an amp-clamp connected to my Pico scope to check the current draw pattern on the fuel pump.  It should be a pattern of eight or so (usually eight) well defined peaks of all roughly the same height.  If one is a lot lower than the other, that indicates a worn spot on the commutator, and if the current pattern is really hashy, that indicates that the brushes are worn out and are bouncing.  Either way I would feel good in recommending to replace the fuel pump as, even if it wasn't THE immediate fault, it's on its way out and will pose a problem in the future.  This was VERY COMMON up to about 15 years ago, on GM and Mopars.  And always replace the relay at the same time, or switch it with something less mission critical/lower time-in-activation like the horn relay.

The first rule is "do no stupid".  It's like the automotive version of the Hippocratic Oath.

If you have no interest in diagnostic work or taking it to a shop yes sell it.  Changing things at random in this situation is bad, you may cause another problem.  But if you do want to take a shot, the basic rule of either fuel or spark works.  If you can check for spark and see it, fuel is the problem.  If no spark, you found the problem.