Amp sound occasional momentary cut out

bremner, assuming it occurs with all sources it would have to be a power cord or power supply issue as you are losing both channels. If all the lights go out it is the power cord or it's connections or a power fuse if the lights stay on it may be an internal fuse or a tube if it has a VT rectifier in it but tubes usually do not act this way it is acting more like an intermittent connection. Start wiggling! 

Mike

Sounds like an intermittent fault in the power stage causing a protection circuit to disconnect speakers or turn off the power transistors. There are relays on the board for the speaker outputs. With the cover off and music playing at low volume, try touching various connectors as Mike suggested and components to see if you can invoke the cut out. Over heating?

Thanks for the advice.  Lights stay on, so i'll check the connections.  I may also put back in some older tubes.  I put in new tubes a few months ago. I would guess if it was one tube it would be on one side only.