"Check Engine" light intermittently on/off?

in my 1997 subaru outback wagon with 2.5l and auto trans (145K miles, new headgasket set about 10K miles ago) the check engine light will occassionally turn on, then after a few days it will turn off. it has been doing this for about 2 months now, with the frequency slowly increasing. when the light is on it seems the idle fluctuates a bit (+/-200 RPM), and there is a small hesitation on acceleration. other than this, there is no apparent difference between when the check engine light is on versus off.

i changed both the oxygen sensors about 10K miles ago, and this month the car passed the local emissions testing without a problem.

could it be a dirty mass air flow sensor? any ideas?

okay, no input here yet so since the "sluggishness" in the throttle is getting worse i went ahead and tried a few things...i changed the fuel filter, cleaned the mass air flow sensor with carb cleaner, and changed the air filter while i had the air filter assembly apart to access the mass air flow sensor. i disconnected the battery to reset the computer, reconnected it, started the car, and the check engine light is still on...GRRRR!!!

anyone with other ideas?

update: the check engine light in my car is now on some days and off other days. i know i have a minor exhaust leak at the engine head on one side, but i am not willing to pursue it if that minor item is the cause of the fault code. since my car passed emissions testing last month i am confident i have nothing to worry about when my check engine light is on!

truth be known i have always despised "idiot lights" and curse the engineer who thought of them AND overall passenger car emission control systems (this means alot since i am an engineer). there are obviously far too many (typically minor) things that can be wrong with an emission control system, but regardless of whether the problem is a 1 of 10 on the "trouble scale" or a 10 of 10 on the "trouble scale" as a driver the only indication we get is an idiot (check engine) light! NOT COOL!!!

thanks for letting me rant and rave :)

Your car is OBDII. You can take it to AutoZone, and they'll pull the codes for free. Meanwhile, go browse over at the USMB. I'm SURE you'll find plenty of answers there.
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By the way I own a '97 Legacy wagon. They really are GREAT cars!

Our 1997 Outback also does that check-engine On/Off once in a while but runs fine. Our car's muffler recently started making noise. I believe it can be from 2 problems:
1. Old fuel-filter ($70) and fuel pump($20) need to be changed.
2. Catalytic Converter needs to be changed after 80,000 miles. Mine is still working at 154000 miles but it's screaming "change me, change me."
I'm changing those 3.
A new NGK oxygen-sensor, a new set of NGK iridium plugs and a set of borg-warner spark-plug wires are always great to add since they improve the gas mileage by up to 7mpg.

Carb cleaner leaves behind deposits that can throw the MAF off, use MAF cleaner next time. Exaust leaks can lead to valve damage as cold air gets to the hot (really hot) exhaust valves, it is something you want to fix before it gets too bad. I'm not sure if that car has a MAP sensor but if it does check that.