Hello all from Boston. 1960 Hydra-matic question:stuck in 1st only after it warms up

Hello, I've been a member for a bit, and find the threads a great help and glad to be with like minded folks. I have a 60 Cad with a 390 and Hydra-matic, I pulled the motor and rebuilt it myself, won't win any awards but runs great (I'm a jalopyist, not into show cars or restoration at all, less $ the more interesting the car to me. If it wasn't throwing clouds of smoke I would have rather not rebuilt it).
The trans was always strong, no problems except leaking. I pulled it during the motor rebuild, resealed the front and put it back in with the motor. I lost my 2nd and 4th gears, so I pulled and cleaned the valve bodies and servo a couple times until everything worked. Now it only shifts when cold, the minute the trans is at running temp it's stuck in first. Anyone ever had a similar problem? Could it be that I put the "reed" that opens and closes with temp in wrong or is it probably the governor? Don't wanna pull the rear off unless I'm sure it's probably the governor.
Thanks. Rod on.