1995 Pontiac Transport backfires through the air cleaner

1995 Pontiac Transport mileage: 145,000. New plugs and wires, new air, gas and oil filters, fuel pump and wiring harness replaced, new distributor rotor, new radiator fan motor, new rebuilt transmission and new thermostat installed about a month ago. The problem--three times at different intervals it loses speed from about 60 miles/hr and coughs and/or backfires through the air cleaner. It coughs, sputters, and jerks until it will hardly go-- about 10 or 15 mi/hr. Have left it at a garage and the mechanics (2) checked fuel filter, fuel pump, and IGN module the next morning and drove it about 30 miles and could not get it to repeat trouble again. No charge as he said he didn’t fix anything but would guarantee it would probably do it again in warmer weather. True to what he said it has stopped again. I left it in a church parking lot over night and drove it home (17 miles) the next day and no problem. About 30 minutes later I drove it one half mile to Wal-Mart and barely made it back home. Check engine light has never come on except when switch is first turned on before starting. The mechanic said there were no codes on computer readout. Hinted it possibly might be oxygen sensor. It seems to act like a timing problem to me but later next day it runs fine.