07 sierra denali 6.0 miss fire

The miss fire is very sporadic. Its only when the truck is first started and until it gets to 210 temp. I can then shut the truck off and turn it back on and its fine. Sometimes this happens multiple times a day, other times Ill go for a week without it happening. The garage said the code just reads missfire on cylinders 5 & 8 everytime. They said GM has a bulletin saying it could be stuck valves from a dirty fuel system, so they have cleaned the fuel system (including injectors) twice now. Ive replaced the plugs and still no fix. It sounds like they want to start replacing things at random until its fixed and I cant believe thats their answer. They are talking about replacing injectors next or maby replacing the intake gaskets???

Any help or direction on what to do whould be great.

Thanks.