[TenTec] ORION shows strange behavior, like drift during TX

[PA5MW, Mark]

The ORION at our clubstation PI4TUE recently showed some issues:

Weekend before ARRL during 24hr PACC contest:
- abt 30 times RX  quiet
- sometimes TX-freq slowly drifted away during transmission by couple of 
ten's (different bands and modes)
- at some point RX seemed deaf for abt 5min and we noticed we  got 
spotted on 6995-ish instead of being in the 7030-7040 range
We had to shutdown each time to solve the issue

ARRL DX CW contest:
First hour in the contest offered same RX anomolies
Changed PA timing settings in N1MM and raised TX delay on the ORION to 65%
No single issue noted during the remaining 46hrs

Last weekend:
Main operator noted during transmission (160m SSB) the TX drift is back 
again


Remarks:
None of the equipment (ORION, PSU, PA, PC's, Winkey, SteppIR) in the 
shack is linked on their ground connections.
Equipment is effectively grounded via their mains connection to 1 mains 
supply group, the several interlink coaxes and the antenna coax 
connections to the building grounding system on the 220ft high roof ( 
huge matrix of professional lightning protection).

Several other coax interlinks have been used, but the current Aircell 7  
(especially the PL259 for that) proved intermittent at some gnd 
connections in the past. Cables have been cut and re-installed.

ORION is an early 2002 (?) model with slightly darker s-meter compared 
to my own 2004 model at home.
Has had the PSU board upgrade with the exact required specced low ESR 
caps in 2008 from Digikey
Defective headphone opamp on front pcb was exchanged in 2011, not a 
simple operational task
DC supply molex connection+25 ampfuse contacts are cleaned and ok
DC suppy is normally set at 15VDC
DC supply cable has a #31 ferrite with abt 11 turns for CM rejection
Amp is an Acom 1000 set for 400W ouput


Anyone any clue about the strange behavior mentioned above?

73 Mark, PA5MW