Brett....I had thought of that at first....
My 140L had slowly lost power especially on the uplines. I removed the cowl on the next flight 15 minutes later and the slow power loss never happened.
The exhaust hole(s) in the engine cover were not large enough so I made them 4 times larger and that solved that problem.
This was using CP 30% Heli fuel.
Try it without the cowl....surely you must have done that already....

Try tearing the pump down and look at the piston on the bottom....I'll bet it is worn out.
If it is....the internal pump fuel pressure will never stabilize and cause goofy mixture changes.

Dave