When you are driving too slowly in third gear (or even in second), your throttle is almost closed. Vacuum will be high, making your engine strain to get enough fuel...which it will not be able to do...which in turn means your engine can not operate efficiently. And this will cause a bucking/jerking sensation as some cylinders can fire and others are denied fuel and fire poorly if at all. This action will naturally be felt in your drive train. Things in your drive train must be really loose to cause this to happen and if you engine is spinning efficiently it is pretty uncommon for this to happen unless you've broken a ring gear or something. Try shifting back to first and raising your rpm to 1000 or so. I don't believe you'll have that problem then.
