Your TPS is taking a crap. The TPS is located on the injection pump, 3 wires go into it. This controls a few things with the trans. Lock up and overdrive engagement will be affected when the TPS dies. A new one is expensive, the most efficient way to manage a replacement is to buy a $5 variable resistor from radio shack and wire it in place of the TPS. Put the variable resistor in the cab somewhere on the dash, so you can control the speed in which overdrive comes in. The resistor needs to be adjustable up to around 5k ohms or so. Unplug the harness from the tps and splice into those wires. Also place a jumper wire in place of the trans relay in the fuse box under the hood. Look around on the web, or google it, there are a few tech articles out there with diagrams and pics of the proceedure.