1997 Bonneville

My Bonneville has 115,000, and around 2000 rpm it surges on speeds from 50 to 70 miles an hour.
It runs fine for a while and randomly starts this engine surge, rpm go up and down mostly on inclines, and the odd time on even roads.
It doesnt last long, or when it happens I just press the accelerator down more.
This happens also with the cruise control on.
Would appreciate and help in correcting this problem.