Randomly my wife's 2011 Equinox will post up a "service Stabilitrak" warning and occasionally shift hard, but isn't throwing any codes, and there's no codes pending. What would be the EASIEST things to check that could cause this? And what do you guys think would be most likely? (they're not usually the same... figure check the easiest things to check first)

Just got this thing Friday, at the time it needed a new intake VVT solenoid, did that, and since it's at 127,000 miles figured the plugs hadn't been changed, was correct... replaced with the GM iridium plugs, those two things got it running so much better... now just gotta figure this out.