[mikethedruid]

You haven't said whether you have replaced the paper condensers, I would start there. I suspect the problem is in #23, or #19a, 19b, or 19c. You also may have a resistor which is changing in value as it heats up, especially #20 or #21. It could also be #28, #31, #34, or #36. First step is to replace ALL the paper condensers if you haven't already. If one is sliding in value as it heats up, it means that ALL of them are not reliable any more. I always replace ALL the paper condensers as well as the electrolytics before I do anything else, even before I turn on a set of this age.