Occasionally stalls at idle

Two weeks ago I took my '03 MazdaSpeed Protege to the dealership for an oil change. Well ever since then I have been having this wierd issue. It has been occasionally stalling when I push in the clutch or shift into neutral. It is only doing this at lower speeds, thank goodness. There really is no rhyme or reason to this: sometimes it does it when the engine is hot and been driven hard and sometimes it does it during a morning trip to the corner store. It just loses RPM’s and stalls out; starts right back up no problem. I took it back to the dealership to see if they had any clue what it could be, but, after a visual inspection, they couldn’t tell me anything. No clue. Any Ideas? Could it be the air filter? They claim to have checked it, but I’m not sure if I believe them. It’s cold air intake and the filter can only be gotten to from underneath the car, which requires me to put it on ramps to check it because I can’t even get my head under there. Could it be the feul filter? I pray not. Other than this the car runs beautifully. Thanks for you wonderful input, as always.

Could it be the Idle control valve (or whatever it’s called)? Sometimes it almost stalls then recovers itself by spiking a little before going back to normal idle speed.