I have a 1991 Ranger 4WD with a 4.o and A4ald auto trans. I am having a problem with the transmission. Everything works just fine until I get up to highway speed, then the transmission can't decide what gear to stay in,keeps changing gears. Fluid is red and full. Anybody ever have this problem,if so how to fix and how much cost? The trans hunts whether the gear selector is in drive or in overdrive.