94 Olds Cutlass Transmission problem?

I have a 94 Olds Cutlass Supreme with 176,000 miles on it. I just purchased it about a month ago. The previous owner said the Transmission had been Rebuilt about 2yrs ago. Anyway it runs great. I have a problem when reaching between 50 and 60mph. The car does a shutter for about 2 seconds seems like when an up shift when going from 50 to 60mph. It also does it when coming back down from 60 to 50 mph. It doesn't happen at the same spot. But it does seem to happen when the accelerator is touched lightly. If I accelerate quickly to say 65mph and set the cruise it doesn't seem to shutter or I don't notice it. I am not sure it the trans needs flushed or if it is something else.