I would quess oil fouling, a big issue with these motors, mine has its moments and I know theres a thread down below somewhere. But your not sure so to condense// try some WOT runs where you are safe doing so, AND please dont use GM'* recommendation to cram car in second gear at 80 MPH> which is the speed I get to for there recomended "technique" RPM.
The rings in this motor take forever to seat and they have to be "strecthed" to rattle at the uppermost region of the cylinder (I know that sounds queer but thats what GM says???) Mine has calmed down after many aggressive WOT and the tailpipes no longer spit ....
Also this is a ten-1 compression engine and I you use lower grade fuels you are going to have problems also. With 7000 miles hard to say it would be anything other than those two items. Plugs are good till 90,000 miles and the diagnostics would tell you of anything major.
Do you have your GM/ Onstar page up and registered?.They send me advanded diagnostics once a month,...and you can choose on demand once your page is set up. Put some premium in it, pour in a bottle of injecter cleaner and WOT, and see if it clears up.
P.*. watch the limiter in WOT,...it can be hit and does some strange things that are definetly not good with chain drive cams and valving that destroys the motor if ya snap one.