GMC Repair: 1991 GMC Vandura Transmission, gmc vandura, shifting into overdrive

Question
 I have a 1991 GMC Vandura 3500 extended cargo van. it has the 350 V8 and I am pretty sure that it has the AT,TH700 trans in it. From a cold start it shifts fine and everything seems good. But as it warms up it stops shifting into overdrive and then at the next stop it wont shift into drive. Then a few minutes later if I slow down or stop it won't shift into 3rd. It just keeps going until it does not shift at all and is stuck in first gear.  This is the same routine after every cold start. I have tried changing the trans fluid and cleaned out the trans pan. No metal chunks on the magnet or burnt smell to the fluid. I replaced the trans fluid with 60 Lucas transmission fix with no change. I also checked for fluid flow to and from the transmission cooler/radiator. Both Hoses are flowing fine. Being a 91 there no codes to check for in the computer. I feel the next step is a run to the transmission shop. Do you have any other ideas or suggestions before I take it to the shop? Thanks for the help.
Jeff

Answer
Hi
Not being a great tranny diagnostician, it does sound like your assumption is correct to head to the trans shop.
It really sounds to me like your loosing fluid pressure or a seal/piston seal or valve is leaking or faulty, maybe sticking.
I wish I could help more.
You MAY want to read the experience of one of the Chevrolet experts, one of them somewheres I read had LOADS of trans. experience, so it may be worth looking into.
I am sorry I could be of no more help.
Good Luck with your problem.
Pawl