rpm vary in 3 and 4 gear with out applying any more gas on 1997 Pontiac Grand Prix GTP

when cruising 50 mph or 60 if i give it some gas the rpm's go up and then slowly come back down before it starts to gain any mph is that transmission slipping or what

Asked by for the 1997 Pontiac Grand Prix GTP
Normal for the rpm to go up when you accelerate this unlocks torque converter and the engine rpm goes up to maintain mph. for this is a lower gear.
If it seems to be different than normal let a mechanic take it for a spin.