One was to narrow down the problem with the rear end is to mount a known good/true rear wheel on it to see if there is anything wrong with thre frame alignment. If it checks out OK, then you know it's the rear wheel you need to dig into. If the wheel just gets to be a pain to get fixed (sometimes they get that way if they are messed up too much from many reapairs, some of which might have been botched up) , you might as well consider getting a different or new one to replace it. It could also be a good excuse for an upgrade. At least you will not have to live with any kind of mysterious problems that the previous owner might have had or caused on the wheel and it will let you start fresh with the bike to enjoy it. .......While you're at it, you might as well treat yourself to a front one too to match the new rear, if the budget allows......

Chombi