727 sometimes not going in reverse

I just did a pressure test on the rear servo and it looks to be in spec. The Guage was 250-275 psi but the needle was jumping back and forth a decent amount? Based on what you suggested above I guess the next step is to drop the pan? I will. Mention again that reverse is not always not working, only about 7-8 times out of 10 does it not work and usually on an incline with nose pointed down. Thoughts?