Hesitation and stalling

I have a 1996 toyota camry, 44,000 miles, 4 cyl.
The car runs great but sometimes it acts as though it's going to stall. Once in while it will stall. This usually occurs while sioping at traffic lights, or when I go up a hill and let off the gas as I get to the top, but not always. I can go for days of weeks without the problem. No check engine light comes on, have used injector cleaner, dry gas, and changed fuel filter. I have had it to a few mechanics but the symtom never occured while they had the car. I was thinking a falty relay. Any suggestions?