I had a problem like this because of my statement above. Do you know if you have a rollerized rear support ? If so, you are lossing gov pressure in the rear lube circuit. The easiest way to fix this is to dissassemble the VB and seperator plate and JB weld around the supply and return gov passages in the lower casting. I done this before but don't have any pictures.
Allan G.

1970 Challenger w/572 Hemi street car and my pride and joy. 1986 T-Type with 272 Stage 2 Buick V6 engine - True 8 second street car. Just updated the engine and put down 928 HP @ 35# boost to the ground on chasis dyno. 1976 Cee Bee Avenger Jet Boat - 460 Ford powered.