1995 Chevy Corsica stalling

For the last couple of months the car will slow down and then stall with losing all power. This happens after the car had been parked and I have to slow down, at a turn or when I come to a stop. I shift into P, start the engine and the car will start right away. After this there is no problem driving the car. When I start the car and let it run for a while before taking off, the car won't stall.
i took the car to the garage. The mechanic told me that bad gas was the problem and added dry gas. After this the car still kept stalling and I took it back to the garage.
then an idle air control valve was replaced. This did not solve the problem. When I called the garage I was told it must be bad gas. I pump gasoline at different gas stations and don't believe bad gas is the problem.
i have to mention that before this all started I had a new fuel pump installed at the same garage.