Miata ISC valve testing - help?

Been diagnosing an off-idle stumble for a while now. I tested the ISC and air valve by running the engine until warm, then connecting the 10 and GND in the diagnostics box. The idle instantly went from normal-ish (850-875) to 500 or so. I then disconnected the jumper cable and unplugged the ISC. The exact same thing happened. It's supposed to jump the idle up to 1200, but it plummeted back down to 500.

I'm trying to follow this article here:

http://www.miata.net/garage/isc.html

but does it sound like my ISC or air valve is shot? Would that cause the problems I've been having with surging below 4k RPM's and laggy throttle response? I've also been getting rather poor gas mileage (5 mpg less than I should).

Did you set the idle back to 850 with the jumper in the diagnostics box? Because that's where it's supposed to be. Your ISC is working, otherwise unplugging it wouldn't do anything. Get the base idle set correctly, then continue to look at any remaining problems.

Poor gas mileage, laggy throttle response - how's your O2 sensor?

In reply to Keith Tanner:

I did not re-set the idle back to 850. I'll give it a try. I have not checked my 02 sensor...that's a distinct possibility.

Keith -

I think it might just be my O2 sensor - car seems to run a lot better today, might be the weather affecting it?

Might as well plop down the 50 bucks to change it. Hate pouring money into something I just bought...grrr