Idle sometimes jumps around, then car stalls

1996 Camry LE, 110,000 miles, Auto, 4cyl.

I'm not a mechanic, just looking for advice on what may be the cause of the following problem. The car was recently tuned up--plugs, filters, oil change, etc. Plus new timing belt, but this did not change the problem described below in any way.

A few months back I had to stop very suddenly, the car stalled on me but did restart right away. It had never done this with a sudden stop before. Since then, whenever I run the car 20-25 minutes, especially on a highway at higher speeds, when I slowdown to stop at a light the idle jumps back and forth around 1000 rpm then suddenly drops and the car stalls. If I keep a foot on the brake and a foot on the accelerator I can sometimes keep it from stalling, but this is getting tougher. It *will* start back up again. This happens sooner and more often during wet weather, after maybe 10-15 minutes, even when the car is parked and sits for a bit at a store and is then restarted. Sometimes there is no problem at all, even with a longer ride, so the problem is somewhat intermittent.

I work from home so I don't drive very much, on the average 100-150 miles a week, and generally stay close to home and so avoiding the problem, but it does need to be fixed so it's safe again and I can do some longer journeys. My funds are extremely limited right now, especially after having the major tuneup done (badly needed, it ran me $850), so having some idea of what is doing this and what it might cost to fix it would also be helpful.