Check all AVC resistors for drifting. Check all RF cathode bias resistors for drifting and/or overheating. Check any resistors that send B+ to the RF plates (either directly or through RF transfromers). How are the voltages at each RF plate and cathode? AVC grid voltages are very difficult to measure properly without a vacuum tube volt meter (VTVM), so don't bother really. As long as the resistors are at value and the condensers are brand new, things should be fine here. I assume that the resistor bank (R1, R2, R3, R4, and R5) is wire wound. Ordinarily this will not cause you any trouble, as wire wound resistors usually either work or open up completely, which would be quite obvious. On occasion, depending on how the wire taps are made, corrosion can develop, causing trouble. If the resistor bank is one of the types that is encased in sheet metal, there is a chance for leakage between the resistor and the sheet metal if the insulation breaks down. Check the former items first before digging in deep. Also check for weak and/or gassy tubes. If you find a weak tube, replace it and see if the trouble goes away. If not, save the old tube. Just because its emission does not meet the specification of the tube tester does not necessarily mean that the tube is bad for the circuit which it is used in, especially if replacing the tube with a new one makes no difference in performance.
Thomas