I don't know much about these things, but for WIW, I asked the guy that rebuilt my powerglide, and right away he said it was likely the govenor. He said it was inside the trans, under a side cover plate? He said it sounded like it was stuck or not getting enough fluid. (I can't remember if you said you had checked the level or not) He also said there are a million powerglides out there and it was a very reliable transmission, and all parts should be readily available.
Additionally, he did not think anything in the engine compartment or vacuum line would cause this. Nor would the missing kickdown be an issue. The kickdown switch and bracket was missing from mine, and I think it was $100 to track down a new (used) one. I have seen NOS ones on Epay go for $200 plus just for the switch. I found mine at Franks Pontiac Parts.
So, for what its worth... (I'm just trying to be the messenger)
Good luck!
I used to be indecisive. Now I'm not sure. I feel like I am diagonally parked in a parallel universe. 1968 400 convertible (Scarlet) 1976 T/A - 455 LE (No Burt) 1976 T/A New baby, starting full restoration. 1968 350 - 4 speed 'vert - 400 clone (the Beast!) 1968 350 convertible - Wife's car now- 400 clone (Aleutian Blue) (Blue Angel) 2008 Durango - DD 2008 GXP - New one from NH is AWESOME! 2017 Durango Citadel - Modern is nice! HEMI is amazing! 1998 Silverado Z71 - Father-daughter project 1968 400 coupe - R/A clone (Blue Pearl) (sold) 1967 326 convertible - Sold 1980 T/A SE Bandit - Sold
