[Mac2002]

I have a 2000 Windstar with a 3.8. 247,000 miles, but only 142,000 miles on this transmission. It was replaced at 105,000 miles in 2005 with a Ford replacement unit, at least that's what I paid for. Now, when I begin a trip from a cold start, I can go for about 45 minutes with the transmission working perfectly. I can put a load on the van climbing hills and the transmission stays in OD like its supposed to. After about 45 minutes, the transmission will not take a load in OD. It will not stay on OD on level ground, though if I hit a steep enough downhill or coast, it drops right into OD until the next time any kind of load is put on it. Then it drops into 3rd. Any idea what could be causing this and is there anything I can do short of a total rebuild? Thank you