ORIGINAL: J-MICHAEL
That was the first thing that came to my mind when I read that was a bad servo. I would re-check em, even brace the heli and trying the test under load also... Also back to the beginning, just make sure your swash is level, and no binding anywhere. Make sure your Bell Mixer Arm screws aren't in to tight. I like to use Tri-Flow and lubricate all moving parts, even some gun oil works great also.
But anyway, it sure sounds like a bad servo.... Be cautious as they will fool ya some times !
Good luck, keep us informed... Any pics or vid of your prob would help tremendously...
~ Jeff
Binding crossed my mind, but servo probs are pretty elusive...especially if he's running the S-75's. Those are digital, and weak...go bad quickly. NOW....a bad motor drawing from the 4-1 even though it looks and sounds OK is possible too, be it tail motor or main motor being bad, it can overdraw and cause servo probs, even burning a digital servo up. Had to put my Blade CP thinking cap on for a minuet.