93 Pontiac Grand Prix

I am having some problems with my car and was hoping someone can help me out. I have a 93 Pontiac grand prix with about 190,000 miles on it with an automatic trany. After driving the car for about a half hour or so that is when the problems start. When I come to a stop it seems like the trany will not shift out of the high gear, causing the car to stall out (like when you do not push the clutch in on a standard). I try to give myself a good distance from the car in front of me so I can slam on the gas before it stalls out and I am able to stop with out stalling. This seems to work. Also, while going up a hill and I step on the gas, the engine bogs down until I apply more gas and then the trans switches gears and I hear this loud thump. I just changed the plugs, wires, and air filter. I also cleaned the EGR valve. I am hoping that it is not the trany since I have a new mustang on order and do not want to put a lot of money in the car. If anyone can give me any advice on what I should try would be appreciated.