Throttle Jams

HiWhen i Do a long flight 2hrs+ and io fly the route using VNAV with the autothrottles armed, the right throttle jams when i Use LVL CHANGE to descend. This problem does not go away even after i disarm the Autothrottle and it jams at around 50% N1. The throttle on the Joystick does not respond at all, but only on the right engine (left functions NORMALLY. The throttle also looses response on the popup panel- to an extent. whilst this still controls the throttle, it is not smooth and accurate possitioning for landing and glideslop capture is mpossible as the power needs to be (badly) adujsted using the mouse, almost impossible whilst flying using the stick.Any Ideas on why it happens or how to fix it??Peter Crease