I also have that problem with my JCM-800 lead series... Somtimes while im playing it will just cut out (especially annoying during band practice or shows) but then i take out the input to my guitar and it put it back in and its fine... this is very wierd and concerns me greatly. I thought it might have something to do with old tubes since its vintage and I have no idea when the tubes were last switched out. But good luck to you Mike.