After playing for about an hour or an hour and a half, the sound from my amp fades away while distorting very slightly. It's the same sound as when I turn the amp switch off while letting the last note ring. After a few (sometimes 10-20) the sound comes back and then after another few seconds it fades away again. I'm pretty sure it's my head that causes this. It's a 200w Marshall Integrated Bass System I bought second hand and this problem only came up recently. Does anyone know what the problem could be?