Tranny Problem
 

I have a 94 Z28 with an auto, about 2 years ago i started having problems with the transmission, nothing serious i lost back pressure on 1st,2nd, and 3rd gear but everything worked fine just no down shifting on the tree. The other night while i was driving home i was amazed to find that the transmission was operating perfect in those gears again, however now i have a bigger problem with O/D, when the car shifts into Over there is a large amount of back pressure almost like a shift down to 2nd when you apply power to the motor it is like there is no power transfer to the rear you can crank an extra 2000RPM and still not accelerate, but everything else works fine. Has anyone encountered this? Or does anyone know where to begin to find the problem? Thanks