My 2005 Grand Prix gt 3.8l rpm jumps up and down while driving while foot is steady in gas pedal what is causing this?

Bad shocks/struts Are your tires have flat spots or cupped ?? Tires out of alignment or out of balance or supendsion parts example being ball joints/ tie rods inner/outer

If, as you say, it is your engine RPM, I doubt if it a front end/tire issue. It may be your transmission is searching for a lower gear to maintain speed. When it goes to a lower gear, your RPMs will go up slightly and come back down when your demand no longer requires the added torque.