If you have been running OS plugs in your head button you have probably damaged the button and it will never seal right with an RB or Nova plug. You might have to get a new turbo button to run RB plugs again as the taper is not the same. The way you describe the engine going from good to bad after 5 minutes is very strange and seems like it might be an air leak of some sort when up to temp. I also agree with Nitronut about getting these engines modded, the C6USA is already a modded engine and when someone is grinding on a crankshaft or sleeve with a dremel it won't take much of an oops to make a stud a dud. I have also seen with most modded motors I have helped people with that the tuning gap ends up very very small, and when they are tuned right there they are amazing but when they are off they struggle. If you are at sea level the C6USA should pull your truck around very easily. I run at 6,000 ft and my USA is a ROCKET in truggy.