Truck derating power after 60mph.

I have a 2006 c13 peterbilt 379 dump truck. About a month ago the truck started derating power and couldn’t get over 50mph so we had it with a mechanic and the code that popped up was for the intake valve actuator number 3 and intake valve actuator number 4. He replaced the entire harness and although the check engine light stayed on, the truck ran fine for about a month but yesterday we had the same issue come up. The truck couldn’t get any power and the last two high gears were missing, so we had another mechanic who actually said it was another harness wiring by the engine he did a quick repair and the check engine light went away but the truck was still having the same issues. The last thing we could think of were the air filters which we changed. The truck ran fine for a few hours getting up to 70 but then the issue came up again, the fuel filters have also been recently replaced as well as the oil and oil filter. I don’t know what else it could be, any thoughts?