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

1 answer
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.