I seem to be having a problem with my SWR SM500 head. Right now I'm playing into Avatar cabinets and the sound is real good. Problem happens about an hour or so into the night. All of a sudden the sound disappears for about 5 to 10 seconds. I've tried jiggling cords, turning knobs but it still happens. Anyone have any experience with this? I should tell you that the amp is in a two space rack and it just fits. Is it overheating and shutting down? I know there are a slew of possibilities here but I thought maybe someone had this problem and could advise me. Thanks.