1984 Chevrolet Van Transmission

I rebuilt the ignition system 3 years ago, and checked for vacumm leaks before I headed out on a three hour drive. On the way back I noticed the vehicle would hesitate around 55 to 60 mph periodically. It got worse as time went on. At lower speeds it didn't do it. I was thinking maybe the torque converter might be getting weak. The trans fluid is red and full and there's no burnt smell. Thank you!