I used to have problems with after market clutches (S-10's)
releasing to early, like yours, or not releasing at all. The clutch plate has a cushining spring between the 2 plates, and sometimes this spring expands to much,(wrong spring) and not allowing enough clearence when released by the pressure plate. A plate that is too thick can also cause this problem.

The older I get, I only want to work on older rigs.