As far as your clutch slipping compared to a diff slip small adjustments to the diff To make the diff tighter should make to noticeable difference in the symptom you are describing. Also if you feel like the slipper is set correctly that it would slip under normal acceleration if it was any looser most likely your diff is the part that is not correctly adjusted. As far as the servo of course check the transmitter to ensure all steering setting are correctly adjusted then I would remove the servo with servo horn installed manually move the servo left to right if it is not a smooth transition then i would say you have stripped teeth. Are you using a plastic servo horn or a aluminum one? Have to keep in mind the quality of servo you have plastic gear, metal gear or titanium geared. Forums are good for direction and advice from more people then you would be able to encounter in the rc world but symptoms like this have many possibilities. What vehicle 2wd/4wd what kind of surface are you running on?