Brightness pot is probably getting a little worn out and the wiper is not making good contact. I have a Monitor II doing the same thing. Jiggling the knob usually brings it back. Could also be old solder joints (easy fix, just reflow the joints).

If the pot is on it's way out, not much you can do...maybe replace it, sometimes spray a little contact cleaner inside (easier said than done) and 'work' the pot a bit to hopefully clean the wiper. I've fixed pots on stereos this way, at least for a little while.