[69Charger]

It sounds to me like the tranny is going into lockup.... That is 100% normal... in 84 even the TH350 had lockup solenoid..

When you are normally driving down the road the torque converter slips the transmission to keep power up. When you get on the highway and get to 60mph and maintain that for x amount of time it goes into what is called "lockup" it is hwere the torque converter locks and puts all the power throuhg the transmission.. This is mostly for fuel mileage and to keep the transmission temperature down.
If you don't mind running your transmission hot, you can disconnect the lockup on most 700R4's by pulling the 4 pin 1 wire plug on the drivers side of the transmission.. but if you don't mind it, leave it alone.. You'll get better mileage and your transmission won't heat up as much!