I have seen this many times.
I believe in a lot of cases the difference between cold not grinding and hot grinding is the temp of the oil in the transmission.
Clearly the clutch is dragging.
In a lot of cases I have found that the problem is the clutch disc is not running true.
I just went through this on a 1941 PU and the new disc had close to 1/8" run-out.
It is always a good idea to slide the disc on the input shaft of the transmission and give it a spin to verify that it runs true.
