I had a similar problem on an Agilent DSO1002A, only it was the time and voltage scale buttons. Spraying in contact cleaner cleared things up right away. I don't even need to take the scope apart, it was enough to full firmly on the knobs to remove them, and then guide the contact cleaner in around the exposed shaft. I gave each knob a few whirls, pushed the button a time or ten, and it cleared right up.
Did the same with the push buttons and they started behaving a lot better, too.