Checking the belts is never a bad idea but when alternators get old, like all things eventually, the bearings go out which could cause the car to jerk.

Might not be that serious though, maybe the alternator isn't keeping the battery charged (whether loose belt, etc.) and that when the bearings snag, the belt slips more than usual, or it simply doesn't put out the correct voltage for the situation, that the car jerks because it isn't getting the spark it needs and the battery isn't strong enough to compensate. :shrug: