[Chas]

Does this volume drop out occur on all bands?

If only on one band, measure the resistance of all the RF/Osc coils. A corroded winding will always have a higher resistance, use the Simpson. Most of these coils will be around 20 ohms or less. If you know which coils are layered secondary over primary. disconnect a lead from each winding and measure resistance of the one winding to the other looking for leakage caused by the layer insulator corroding the windings. It should be infinity, use the high ohms scale so 9-volt battery is in play in the 260.

I have found several strands of litz in an IF coil open at the solder joint. Connecting all of them lowered the resistance and improved the "Q" so the winding could peak properly.
Thus, another clue is a alignment setting that does not peak (or null) as expected yet works. Indicated an issue with that circuit.

If the volume drop out occurs in all bands, concentrate similar coil measurements at the mixer/IF circuits right up to the volume control.

Another alternate method is to use a spot heat gun or a freeze spray and methodically try to either induce the issue or clear the issue. Keep in mind that separating the AF from the RF at the volume control helps clear one of the three sections of the radio, the others are the RF and power supply.. The Candohm, if still present and the output transformer primary. The output transformer bypass cap and tone circuits can be problematic in this model.

New components can be faulty, as I stated failing "Shoopage" in a modern poly cap and intermittence welds in modern resistors.

But stage isolation must be done or its just a crap shoot to find a bum component.

YMMV

Chas