You can get a drop in tail rotor rpm without getting a drop in main rotor rpms. You need to go over the entire tail drive system and chack that all the gears, pulleys, belts, shfts, etc. are secured properly. Look for loose set screws, those things can work loose if not loctited.

Even though you sent the servo in for service, you might open it up and make sure all the gears are OK. If you've got a tooth or so missing, you could have intermittent slipping in the servo.

Ben