Trailer tire discussions are usually based on "this is what I do" and since folks take different approaches, must be possible that there is no absolute right or wrong way to inflate. The important issue is that the OP said he had D load range 15"s and these are 65PSI MAX pressure per Goodyear spec. His trailer is almost exactly the same dry and gross weight as mine and I use E rated Carlisle 15"s and run them at about 80PSI and have no whatever the OP called it... my trailer pulls good. Suggest perhaps some other issue since tires can be out of round from the manufacturer. Perhaps balancing is the answer?