1995 Chevrolet Beretta

Transmission problem
1995 Chevy Beretta 6 cyl Automatic 150000 miles

I've got a 95 chevy beretta. 6 cyl, 3.1 L engine. Recently, I've had some problems with the transmission not pulling off. I start the engine and when I put it in drive the car just sits there. If I don't have rev the engine, the car may sit there for 10-15 minutes before it'll pull off. I changed the fluid, filter, and gasket a couple weeks ago because thats what the mechanics I took it too thought that was the problem. However, it didn't fix it. What else could be wrong?
November 5, 2009.

If it doesn't engage, make sure the trans module is good and the wiring to it is tight and clean. However, it seems to me there is internal problems. Chances are it is in the valve body. Has the check engine light ever come on?