I heard a sizzle type of sound one day. I didn't think that was a good thing so I turned the amp off.  The next day I began to play and once it got warmed up, sound stopped coming from the speaker. I can shut it off, let it cool down and it will play once again for a few minutes and then it stops again. I have owned the amp for a couple of years and never had this problem before. Is this a common problem? Suggestions?