Help--intermittent stalling problem

I have a '91 Legacy that's been stalling on and off for 10 months now. the problem came on suddenly last summer--and the car stalled frequently for several weeks. It always starts right up though.

Had the fuel filter replaced--the mechanic thought that was the problem, but no, it stalled again 5 minutes after leaving the shop.

Over the fall and winter months, the problem nearly disappeared--the car would stall maybe once a month, usually as I'd come to a stop. In the last couple months, the car's stalling much more often. It seems to definitely be worse during hot weather.

The engine usually stalls on start up after it's been shut off for 10-30 minutes (engine is still hot). It also stalls when I'm coming to a stop. Lately, it stalls while I'm driving. Very sucky.

The other day, it stalled on a "cold start" for the first time since the problem started last July--but it was a very hot day and the car was sitting out in the sun.

One other thing--several times after having trouble with the stalling, I've opened up the hood and knocked on the mass air flow sensor--and the stalling stops.

any ideas of what else could be causing this intermittent stalling? Especially anything related to temperature?

any help would be appreciated....

--Faith

Have you changed the air filter?