The stumble could be related to a fuel delivery problem. It's worth checking the fuel filter for any clogs or restrictions that may be affecting fuel flow.I took the QP GTS on a driving vacation a few weeks ago, and noticed something that has not been obvious 192.168.100.1 192.168.1.1 to me in my daily commute. At highway speed at constant cruise, the car has an intermittent stumble. The frequency of occurrence varies, and there were stretches were it was very infrequent and others where it was happening within 10 second intervals. Car accelerates smoothly and I don’t notice it around town. It is pretty minor, so any road imperfections mask it. Only notice it on smooth tarmac while cruise control is on under constant engine load. I bought a fuel additive and tried a few full throttle accelerations to clean things out, but all i could notice is more or less frequent.
Any ideas where to look first?