My aging 26" Nishiki 10 speed is still a great bike and ride and well maintained. 25yr warranty ended in 2009. I am 6'7" and 280lbs now and was 250lbs when I purchased it. For some strange reason now when I ride the back wheel pronates over left and rubs against the back brake pad and the fork. The moment I get off it's okay. What gives??? Has anyone had an issue like this?