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I'm not an expert troubleshooter, by any means, but I do know that those kinds of intermittent problems are the most difficult to isolate.
You didn't say much about the radio itself. Is it still running on all of its original components? My experience has been that any radio more than several decades old is living on borrowed time, as far as its capacitors are concerned. Failed or failing bypass caps in the RF and IF stages are notorious for causing oscillation problems. If this radio still has any of its original paper or electrolytic caps, you would be time and money ahead to replace those before going any further. This can either solve or prevent a whole slew of problems.
Also, check for loose or ungrounded RF or tube shields, broken or intermittant ground connections, or wiring that may have been improperly dressed during previous servicing. Less likely would be a tube with an open supressor grid. This won't always show up on a tester, so it's best to substitute with another tube.
Just a few ideas for you. Someone else may have more direct experience with this particular problem.
Good luck!
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