re solder all connections on pc board but we have seen
the speakers in these amps fail frequently.
check the pigtail leads on the speaker basket, damaged
leads from loud playing can cause what you describe,
intermittent output.
play the amp really loud to trigger the symptom, with a bass.
or tap on the pc board with a plastic handle to find evidence
of the bad solder connections.
eden sells the replacement basket which bolts to the
magnet. easy field replacements.
the valve is ECC83.