95 GP...Downshift problems

My 95 GP has just recieved and trans transplant.Now here's my problem.It seems it wont down shift @hiway speeds. rpm's will drop to 1800-2000 when i get worried.I can let off of the gas then reapply part throttle.It'll down shift then drop right back to the 1800-2000 rpm again.Could it be a vaccum leak, or something more complex?

I got it fixed......I had a bad spark plug wire...put on new set of wires.Car run great now