Is it the amp with the EL84 reverb driver, or the later 6V6 type? The EL84 type gives loads of trouble & weird noises thro the reverb, though not oscillation I don't think. If it is an el84 see if twisting the dwell control affects the oscillation, just to get the reverb issue out of the way.
Microphonic preamp tubes can cause oscillation, as can a bad preamp filter cap. Check for these by tapping preamp valves, and by (cautiously) bridging the filters with a good one. C34 and 35 are prime suspects.