The belt should be running true, So something must be out of alignment. Check the tail boom to make sure it`s not bent to one side or the other. I have seen this happen a couple of times, when the tail boom is slightly bent to the left or to the right.
Also check the belt itself, for abnormal wear. Like wearing on only one side. Also, if the tail box is two plates.....like on goblin helicopters.......that you slide fore and aft, to adjust the belt tension. Make sure that they are adjusted equally and that the tail output shaft is nice and square with the tail boom.
These are the usual suspects when the belt dances around like that. I hope this help you get it sorted.
Belt tension wouldn`t?shouldn`t cause this to happen. If the belt is too loose, it will just slap the inside of the tail boom, and slip on the pulleys. Which will just decrease your tail performance.
Having the belt too tight will cause side load on the bearings and potentially cause premature bearing failure.