2005 Envoy XL - At Idle Loping Felt in Seat

I just bought a 2005 Envoy XL with the I6 and it drives great. However when stopped at a stop light and I take my foot off the brake it acts like it wants to surge a bit. Sort of like if you were to tap the gas every half second or so and let off in between taps. I don't see the tachometer rise above 650 rpms where it should be. Feels sort of like tapping a clutch. The air filter was recently changed. At 110,000 miles I know I need to change the spark plugs so maybe that's it. Has anyone had experience with this issue?

Do the plugs and clean the throttle body also.

Do the plugs and clean the throttle body also.

No change. I changed the spark plugs, checked the coils, cleaned the throttle body and changed the air filter. The vehicle still feels like it is moving up and down or back and forth a few inches while taking my foot off the brake. Feels like I am riding on a rough road in a big rig sitting on those air suspension seats. Any ideas what it may be?

From it is impossible to diagnose this accurately, have someone knowledgeable feel what is doing to get a better idea.