Some easy Tranny checks.
Make sure the vacuum line from manifold to modulator valve on the tranny is good w/no leaks.
Pull vacuum line off of modulator valve and check for trans fluid. It should be dry, if fluid is present the valve is bad.
Make sure the modulator valve is tight in tranny.
What fluid are you running in the tranny?
Make sure linkage is properly connected at tranny.
Does the tranny have the kick-down linkage connected?
Good luck
Dave