Transmission goes to neutral @ 70 mph
 

I have 2001 Dodge 2500 with the cummings diesel engine. When driving at highway speed 65 & higher the transmission acts as if it has shifted to neutral and and the tac revs up to 2500 and I let off the pedal and as the truck slows the transmission will re-engage. This will happen every time I go above 70 mph. Last night the truck seemed to have an electrical hick-up and the doors unlocked while driving down the highway at 65 mph. So far I have rebuilt the alternator thinking I had an electrical problem because I was told I have a fluctuation in the voltage. I have cleaned and reconnected all the ground cables and wires I can locate. The transmission shop says there is nothing wrong with the transmission. I have replaced the throttle position sensor and performed the the reset procedure for the sensor twice with no improvement. Any help you can offer would greatly be appreciated.