[Chuck Schwark]

All all the resistors replaced? Even if the old one(s) checked ok, after heating up a bit, carbon comp. resistors can change value, especially the old types. How about that three-section resistor #61? Looks like higher wattage types there.

Could be a bad/cold solder joint anywhere - tube sockets, term strips, lugs on parts. Maybe a notorious Philco coil going open in RF section causing low gain on front end.

You need to make a chart of the tube pin voltages when it's mis-behaving to compare with "working good" condition. That will point you to the troubled area.

Chuck