If tube swapping doesn't fix it then your next step is to check for adjunct components such as the resistors you mentioned. They may have drifted enough in value to cause trouble. If it tends to happen after the set gets hotted up then I would tend to want to blame a capacitor. You may have recapped the set, but it isn't impossible to have a baddie in the new lot. In replacing frequency-critical capacitors you need to be sure that aside from its mF and voltage ratings that the new one has the same temperature coefficient as the original. If the device you put in has a "temp stable" characteristic, and the original had an "N750" designation then you could run into trouble after warmup. Caps with such coefficients were used to compensate for the natural tendency of a circuit of given design and component choice to drift out of tolerance.
The other thing you might want to examine is that the aquadag coating on the CRT (if this is applicable) may have become ungrounded. A horizontal sweep pulse can electrostatically emanate from an ungrounded CRT and raise hob with sensitive AFC circuits.... just a thot....
