1997 Chevy Cavalier 2.2 100,000 miles

Cavalier keeps dying when idling or moving very slowly in traffic.
Mechanic can't find the cause, can't duplicate the problem.
Power just cuts out- everything else still works (radio, air, blinkers). No lights on dash come on.
Sometimes restarts right away, sometimes won't start again, towed to shop, starts upon arrival or will start when pushed to side of road.
Shop replaced fuel pump and relay because he said when hooked to gauge it lost pressure - car died again next week.
Shop can't duplicate the problem again.
It seems to have happened each time in the rain or damp weather. But, has driven in wet and dry weather in between incidents.
It has happened both when car has driven a distance and in my driveway at start of day.
Any help is much appreciated - it's back at the shop now and he can't find anything or duplicate it - and I'm afraid to drive it- no tows left on AAA! Thanks so much.