While it is very likely that as everyone says, you are either experiencing "binding" or the servo is prevented from full movement, because the linkages cannot move any further ( you can correct this by adjusting the endpoints via the TX )....

... you also may need to consider that your servo or system is getting RF interfernce from the engine.

Are you using mechanical metal linkages and pushrods from the engine to the servo?

These can act as antennas directing engine RF noise towards the servo, and causing it to glitch.

I've also had a servo or two that would do this even with plastic linkages, when the engine was within a specific RPM range.

The problem was with the servo itself, as changing it out for another of the same brand, make and revision level, solved the problem.