My 2001 F150 Supercrew with 4.6 and 74,000 miles has started unlocking the torque converter in OD at speeds 45-55 MPH. It will just jump up a few hundred RPMs for a couple seconds and lock right back then jump up again even with the cruise on on flat ground. In drive even towing my bassboat it stays locked in and at interstate speeds 70-75 MPH it stays locked in OD. Fluid is full and good changed about 14,000 miles back. I just had it flushed though should I pull the pan and change the filter or is this torque converter related? This thing has run like a watch for 74K now the first problem is something I know very little about an automatic tranny. Help!