Any help my 600 cfi runs great out of the hole. But then as you get going for about 3 miles or for a while. the sled when you punch it revs out right but then falls off to 7500 rpms and than occanisical pulls up to the right rmp. took it to the deal and they hooked it up but showed nothing for codes. any help would be great.

Check your drive clutch the bushing in the moveable sheave is probably coming out are maybe is out. It happen to me about a month ago. Hope this helps!!

mine had the same problem, turns out my belt was too loose and streched, got a new belt, adjusted the delflection, now it still only gets 7800

Have you checked the power valves?