Sounds like a parasitic oscillation, do you have any pics of the chassis, tube socket wiring, pot wiring?
If you can make it do it again, perhaps when the neighbours are out, before you turn it off check the dc voltages...if it's oscillating these will be pulled down compared to idle.
Shorten &/or shield your grid wires, even those to the power tubes.
Possibly an intermittent short on the OT? What plate voltage/current were you running?