You might want to try the most simple and chepest first. A hot tune up and bigger exaust is great but you have to get the fuel there and the duramax computer monitors the fuel pressure. I had the same problem; I could only run 1500 rpm and the check engin light was on. A new fuel filter and code re-set fixed my problem. I do and recomend to everyone to replace the fuel filter every 20 tho. mileson my 05 lly duramax when i put i on a hot tune such over 200 hp setting and over and run it to hard it will limit me to 2000 rpm's,but i can reprogram it to the same setting and it is fine untill i run it to hard.my question is what causes that and what can i do to stop it,with out dangering the truck to much.