Any jerking or hesitation indicates a worn idler tire. There is a lot to be said about idlers and reel tables for transporting the tape. The reel tables actually make the connection to the tape reels and the idler is what drives them. The capstan is the only other tape drive component in the transport system. Reel tables rarely ever fail.
Idlers are a little more famous for trouble.
Newer gear driven idlers are having problems with warpage and cracked spring retainers, while the older rubber tire idlers are having problems with the tires themselves. The rubber on them eventually hardens causing a loss of traction. This will eat tape. A servo is any motor that can send feedback to a micro-processor for analysis. The analysis results in voltage correction for the motors power source.