[Amps] More instability problems

[N1BUG]

Recently I asked about an instability problem after my 10 year old 
HB 4CX1500B amp suddenly started destroying vacuum relays. Thanks 
for all the advice! I discovered under certain conditions it was 
going into LF oscillation around 400 to 450 kHz (with lo load 
connected to the output).

I'm afraid there is more going on than originally thought. I used to 
be able to keep a table of tune/load presets and never have to vary 
from them unless my antennas were iced up. Now it seems the settings 
are constantly drifting. I have checked all connections in the amp 
and antenna system. Could this be a sign of tube or vacuum variable 
failure?

On the higher bands I am also seeing instability when tuning. While 
adjusting the tune and/or load caps, I'm seeing sudden jumps in 
antenna SWR and power output. Obviously this is a stability / 
parasitic problem. I am unable to determine if it is the same LF 
oscillation since there is so much RF at the desired frequency under 
these conditions.

Any new thoughts based on the above? This amp had been trouble free 
for 10 years prior to the onset of problems a couple of weeks ago. 
Something has clearly changed.

73,
Paul N1BUG