It's what we techs call intermittent. This could be a loose connection caused by any of the following:
1) Guitar cord into jack - wiggle to see if it's the culprit.
2) Dirty volume/tone control pot - turn rapidly back & forth to try. Clean with electronic cleaner for switch/pots to cure
3) broken/cracked solder joint - would need solder touchup to repair.

From what you're reporting, I'd say 2) or 3) is most likely with the treble potetiometer. Try spraying with cleaner (amp turned off) and sweep the knob back & forth to wash the solution thru the pot.
Moving on.....
did the #2 not sure about the #3 what should I repair? I know how to solder..
Assuming it's the treble pot that is still funky. Open the amp up (after unplugging AC Cord from outlet) and inspect the solder joints where the three legs of the pot are terminated on the PCB and see if any one solder joint is cracked, or "cold" (not smooth or shiny) or even just barely soldered. Sometimes during manufacture they don't apply enough solder and you end up with a solderpad hole that doesn't get filled with solder and the component later breaks away. Touch up the joints by applying heat with a clean iron, (preferrably removing the existing solder with solder wick or solder sucker) and resolder.
Good luck!
Moving on.....