I have BeoLink 3500 connected to BeoSound 9000 and BeoSystem 1
BL3500 is in dining room and ML cable runs to the main room where it's spliced to connect both BS1 and BS9000.

These are problems I'm experincing:
After playing CD or Radio for longer than 15 minuts (changing Trackes CD's etc) BL 3500 goes in to stand by mode, clock is gone and replaced with ": ."
When I play with sound settings of the BL 3500 such as loudness, bass etc it switches off BL3500 to stand by almost immediately.
TV source from BS1 seem to play long enough and never shuts off BL3500.

What could be the problem? Any ideas?
Initially I thought it's a cable problem, so I replaced CAT 6 cable with original ML cable (15-20 meter)
Still have the issue

Have you tried to connect only one ML cable from the Beolink 3500 to the Beosound 9000, leaving the TV out of the loop. 

p.