Will not go into 1st when warm

Bought a Mustang as a roller with a TCI built C6 and it works flawlessly until it warms up where it will not go into 1st. It will just stay in 2nd or 3rd. It has a manual reverse VB and a large cooler with a fan. The shifter was adjusted. It has that nice deep cast aluminum pan. I am not a fan of TCI. Thinking about thermal gunning the pan after its warm.

Any ideas why no first gear when its warm?

Thanks,
Dave

Since this is a TCI transmission it is likely a valve body issue. If you remove the spools and polish them to a smooth slide in the bores it will probably fix your problem. Chamfering the end lands before the polishing will keep the bores from getting galled by the spools sharp edges. Use a dab of Vaseline (R) on the spools when installing them.
As always keep everything absolutely clean.