If a factory reset did not clear the problem, it is going to have to go in for service or be replaced. Anything you might possibly have done, short of physical damage, would have been cleared by the factory reset. The factory reset loads a new copy of the operating system into flash memory from ROM.
If you are not already a member of the PSR-Tutorial Forum, go over there and register, then post your problem and that you've only had it for two months in the "PSR E Series" section, and ask for instructions on contacting Steve Demmings. He's the Yamaha Rep. Most of the old timers over there know him personally. Several years ago, when they had the display failure fiasco on the PSR-S900's, he was helping folks get warranty replacements for keyboards that were a year or so out of warranty when their display finally failed. If anyone can help you with your problem, it will be him. After two months, I doubt that the retailer where you got it would be willing to take it back, unless they have a 3, 6, 12, etc month warranty policy.
http://www.psrtutorial.com/forum/index.php