I have had this trans into a shop a few times and they dont seem to be able to solve the problem.
When its cold it will shift fron 1st to second just fine. But will not shift from 2nd to 3rd until its been driven a few miles and the tranny get up to temp.
From there on it shifts fine, with no problems. The mod valve has been replaced and there are no vac leaks. Any help would be appreciated.