Engine rpm's drifting up and down!

I have noticed on my 98 silverado k1500 that when I am going at speeds of 43 to 49 mph and also 60 through 70 mph that my engine rpm's seem to drift up and down only about 200rpm's. This only happens when going on flat roads or slight inclines. When I press down on the gas a bit more it does down shift and stops fluctuating. What could this be? :screwy: