[R-390] AGC Problem - Diagnosis Help Appreciated

[AE4CW]

Here's the progress so far on my recently aquired R-390A.
 
The BFO tuning is resolved, and I'm now tackling the AGC.  Here are the
symptoms:
1. AGC works on strong BC AM in Fast and Medium with only slight change in
carrier level ~ 80 db.  On Slow the carrier meter goes to 100+ and AGC is
lost.  Moving from Slow to Medium results in about 1-2 seconds of silence.
RF gain for all the preceeding at maximum.
2. AGC does not work (marginal at best) on CW and SSB in any position.
Seems equivalent to MGC.  Perhaps linked to BFO?
3. MGC is very touchy on CW and SSB.  Goes from no signal to overload in
about one dial unit on the RF gain.
 
Here's what I've done do far:
1. Exchanged V504, V505, V506, V508 and V509  with new tubes...no change in
symptoms.
2. Deoxit on all IF deck connectors.
3. Checked leakage of mechanical filters...greater than 50 megohms from pin
6 of IF deck connector to ground.
4. C551 shows nearly 50 megohms.
 
Next step planned:
1. Check IF deck resistances.
2. Check AGC caps.
 
I would appreciate any guidance on likely causes and other diagnostic steps.
---
Chuck, AE4CW