are you cranking the bass on the eq? this could do it. does your bass have a low eq knob that you're turning up? this could be the culprit, too. are you bouncing the strings off your pickups? third guess. any of these could do that. also, if it just flashes a little, don't worry too much about it. if it happens with every note, just back off the master vol a bit. if it stays on, something needs to change asap.