93' f150 5.0 auto has always felt overly sluggish around 55mph in overdrive till ya hammer the pedal and downshift and get the revs up. Always kicked back into overdrive around 65mph till recently. Now I get a shift around 50-55 and then it sticks in that gear till I redline. Feels like it wants to shift to overdrive but never does, very scared to keep pushing it down the highway everyday to work at 65+mph. Any ideas on what might be keeping it from shifting? Started when it first began to get extremely hot outside this summer, lots of 100+ days so far....