[Eliot Ness]

With a problem this intermittent it will be difficult to find, but not impossible. When the problem rears its ugly head use a non-metallic object like a chop stick to poke every wire and connection you can reach (above and below the chassis). That set uses pretty common tubes and I'd just replace them all to rule them out. If that isn't it put them in your parts stash for later use. I'd have to do a lot of wiggling and poking before I started rewinding coils