I agree, it has to be one of the two. Fortunately I won't have to figure out which one it is because GM is giving me a new truck. They let me have it for a few days before I sign the papers this week and so far it has been flawless all the way up to 85mph. Hopefully it stays that way once I take delivery.
I put one of those on a truck...two trucks ago. I wouldn't do it again. It made the truck very loud from the engine compartment, and I never did notice a difference in performance or gas mileage.