Yes the engine has a "lumpy idle". It's aV8 !!! it's a performance V8 !!! It's not a Buick. I'm not being a smart %^& well maybe a little. But the bottom line is there is no crossover tube in the exhaust. That helps balance backpressure from side to side and also lessens a lumpy idle. But you lose that old time exhaust note when a crossover is added to any exhaust.The GTO tries to bring back the old exhaust note of true dual exhausts. Also the LS1 does have a performance CAM. More overlap than a "BUICK", or any other common sedan. More overlap at high RPM is a good thing, at idle it makes the idle lumpy. I remember old high lift cams that if you could keep it idling within 300 rpm it was good enough. GTO's are great cars, BUT they are also MUSCLE CARS make no mistake. If you were expecting an AMG Mercedes type muscle it is NOT !! It's refined and has real suspension ( I'm somwhat suprised someone isn't complaining about the ride ) but it is going to let you KNOW it has a V8 performance engine under the hood. Many folks may have forgotten what a Muscle cars was or maybe many never did know, but I remember and the GTO is a very nice muscle car AND the LUMPY IDLE is part of it's Character.(Beach Boys said " Iv'e got a fuel injected engine sittin' under my hood.")
We have become so accustomed to the modern engines that have been refined so you don't even know the engine is running, if that's what you were expecting you may have purchased the wrong car. LUMPY IDLE is common in all GTO's.
And from Ronny and the Daytonas "When I turn it on, wind it up, blow it out GTO."