my 645 did the same thing when I had it on my truggy. on the truggy it would return to center, but it would pause about halfway through going back and then continue on.

Personally I am done with hitec servos. I fried the hs6975hb that i had controlling my throttle on my truggy. It started making that whining noise like it had a load on it, even when the servo horn wasnt on the thing. then there was a nice little puff of smoke and the smell of fried wiring. I am done with hitec servos.