I reviewed the Sticky post "Quick troubleshooting guide for Dynaco tube amps" and 13 (below) seems to relate, but in an opposite manner.
13. Problem - One channel cuts out completely or drops in volume after time (as the amp heats up)
Areas to check - Bad solder connection, bad solder trace on driver board, bad coupling capacitor
When I first got the amp about a year ago, the driver tubes were so loose in the sockets that I got some loud honking, so I would manually adjust the tubes until contact was consistent. Therefore, I thought the sockets were causing the channel cut out problem. I replaced the driver sockets last night, but the issue remains.
While I had it open I took the mono switch off line, replaced the RCAs, the 8 ohm speaker terminals and I padded the inputs with series resistors to coordinate with my preamp (see: http://www.bottlehead.com/smf/index.php/topic,626.15.html post #25 for details on the padding) and checked around for loose or poorly soldered connections, re-soldering several randomly. All the traces on the board remained attached.