Didn't you read duke37's reply?
As he said, it could a dirty switch. Or maybe dry joints on a switch. Could be the A/B switch or the 4/8-ohm switch, or both.
To find out which, put a continuous signal through the amp and tap or gently push each switch in various directions, to see whether one of them is intermittent.
Or open it up, inspect for dry joints, and clean the switches as duke37 described.