I've had a DSL40c for a year or two, and it worked fine until a few months ago.  While playing, the sound cut out and there was a slight burning smell.  I replaced all tubes and pre amps and it worked fine in testing, but then did the same thing when I got it home.  Repair guy thought it was a pre amp issue so changed those out again.  It worked fine in testing, and then did the same thing again when we got it home.  This time when we turned it back on, it would work, but sound would go up and down sporatically with some popping and crackling.  Replaced the tubes again, and it worked fine for an hour.  Then the sound cut out, and we noticed it would not switch channels.  After turning it off and letting it cool down, we turned it on again and it worked fine and channels would change.  I'm not sure what to do at this point.  My weekend routine of dropping the amp off and then getting it back (with high-hopes) has gotten old.  The problems have occurred while plugged straight into the guitar, as well as through a pedal board.  Also, the problems have occurred via multiple power sources in different locations (i.e. not always through the same wall socket).  

Thanks for your help