C4 won't shift into 3rd gear when cold

Glad you got it atleast back to working warm.

You did say there was a B&M shift kit in it. Maybe it was making more pressure than the TCI.
In any case i would give TCI a call and tell them about your wimpy valve body. ( maybe talk them into a manual )

I am with Paul i bet your seals are worn or hard.
It should get you thru the summer if it will still bark the tires.
Might try some lucas tranny treatment.
It has worked great for me on old hard seals.

Had a buddy with a 93 vert AOD that sat for 3 yrs. When he started it finally it would not shift at all. I added the Lucas in it and when it warmed up he drove it off.
Won't last forever.........