1995 Pontiac Sunfire 108690

I take a 120 mile drive home every weekend, After about an hour of driving my car turns from Dr Jekel to Mr Hyde. I travel at consistent speeds of 60 MPH. When I come to a red light however and my car get under 10 MPH the engine will shake violently and the car will cut off. I can restart the car immediately however as soon as I drop in in Drive (the car is an automatic) the car will cut off again. Almost like it stalls. I noticed a leak in the coolent and took it to my mechanic. He replaced the head gasket, oil valve cover, changed my oil, replaced my serpentine belt and my tensioner. And my spark plugs. He explained it was oil leaking from the valve cover leaking down into the spark plugs. I got the car back and it worked fine I was please. Until my next trip, the car stalled out as it did before, when in a low gear. I consulted my mechanic again and he told me to unplug a blue and white connector from my transmission, the car will run fine and I might notice a faster loss of gas. Since then I have made 3 trips without the car having trouble. Last night I was driving and the car acted like it ran out of gas, I was driving and the car gas pedal wouldn't work anymore. When the car drifted to a stop I shut it off. The car would not start after that. I checked for leaks made sure I have coolent, the engine would not turnover. I called a tow truck through AAA and was waiting for maybe a half an hour. I tried to start the car again and it started right up and I drove homelike nothing happened. I cancelled the tow truck, but I have no idea what is going on with the car, Would you please Help!