I replaced the motor and tranny and didn't put the vacuum line on to the tranny because I had trouble finding the pre-bent original line and I wanted the original line on the car.
As soon as I found it today I put it on.

When there was no vacuum line, It shifted into 1st and reverse strong when I wanted to move the car in and out of the garage.

Now that the vacuum line is on ( intake manifold) I drove it for 10 miles and everything shifts fine except now it slips in reverse.
Too much vacuum?

 

When I put in my th350, my factory vacuum line was too small (narrow) so it caused my tranny to not shift out of first, and reverse would randomly slip in and out. I put in a bigger vacuum line and the problem went away. I would definately mess around with that vacuum line. Sounds like not enough vacuum???