I had similar probs with my '93 b/on when in active mode, vol pot became scratchy & cut out intermittently,cured it by using a "bucket " load of switch/ contact cleaner, warwick pots are difficult to squirt cleaner into ,I unbolted mine & squirted cleaner into every orifice I could see, keep rotating pot while you do this & also pull switch in & out, took several attempts to do this, worth the effort tho' as the old thumb has been trouble free for over a year since I last did this.
WARNING on no account use WD40 to do this,if you don't have electrical contact cleaner, sometimes known as switch cleaner, you can buy something suitable from Maplins, best of luck, mike
I'm sure you checked the battery first tho,also make sure the battery clip is secure on both the contacts of the pp3, have known them spread apart allowing the contact to make & break with vibration, can't think of anything else.