There's something defiantly wrong with those tractors. That's the same engine as CASE-IH runs in 71, 72 and 89, 50 models. Those tractors have plenty of power without the varying rpm's. Find the signal line for the auxiliary pump, cap it and plug the hose. This should tell you if that's the problem or not. You might talk the CIH dealer into looking at it also because they're very similar them.