I have noticed that my tachometer is fluctuating up and down by 500 rpms while cruising at various speeds on the highway. I can hold the speed constant at 55 mph, 60 mph, 65 mph or 70 mph and you can feel the engine kick in as if you have tried to accelerate. The tachometer will go up 500 rpm'* for 3 to 5 seconds then back down for 3 to 5 seconds. This is while driving on flat ground for several miles or while driving over elevations and depressions in the road way.

Any clues as to what might be causing it?

What is the vehicle information? Year/make/trim level....

it is a '97 Chevrolet suburban, 2500 series, no power windows or power seats