With what you've done/tried, the birds nest of ac/dc wires may be the cause.It's a bit of a mess - 120Vac line voltage wires running right next to Vdc wires running close to 6Vac wires - all on two sets of criss-crossing seven lug terminal strips. When I get the new output transistors I'll see if I can try to clean that area up and keep the AC wires as far away from DC wires as I am able.
General rules are that AC active and neutral wires should be twisted together, something about cancelling fields. And DC voltage wires should be kept
as short as possible. You might have to rewire the birds nest using these general rules. Maybe someone with more experiance will chime in, fresh ideas welcome.