Just my two (2) cents, but it sounds kinda like a CSJ (cold solder joint). I've had a few tube radios more than one amp that would tune up "correctly" w/it was warm, but let it cool down 20 min.+ & come back to "NUTHEN". I've learned a lot when it comes to high voltages & decades old "Boatanchors". Of course, I haven't wiggled components on a RF deck, PS, etc., capable of 14 Kw!! LOL