Steering, my UP sometimes pulls to the left

Hello guys! I’ve had an UP since July 2012. Right after the first kilometers the good sometimes started to pull strongly to the left. But he only does this once in a while, then for weeks no more. Of course, the spook is over every time I’m in the workshop. Has anyone ever made an appointment with this fenome before? Or does anyone have an idea where the problem could come from? Greeting Manfred