My personal test bench method is using a Z500 brick, Z controller, and a set of alligator clip leads to the center roller and a truck chassis grounding point with the engine on it's side. If it passes that test with no glitches- then this is a track and pickup roller related issue 90+% of the time.
Again, a used engine, with unknown lube status, something as simple as using the wrong thick oil on pickup rollers has been enough to cause poor or intermittent power problems.
