Help!!!!!

[kiwi1313]

I need someone to please help me, I replaced the engine and tje Transmission on my 91 G20. Perfect match on both. I have one very big problem, the transmission when cold runs great, but after 15 min or so it stops shifting we've taken it out twice to check the inside everything inside is new. Teh baffling thing about this tranny is when it's cold it runs great but once it heats up it stops shifting from second to third and overdrive. Then we checked the Solenoides, both been replaced they are good and are working properly when cold. Once the car heats up they stop receiving the appropriate ohms (25). Put it on a diagnostic machine it's not giving any information about the tranny. Everything else comes up normal. Someone please help me my car has been in the shop for six months, and I can't continue