My Raspberry Pi B+ is working well except: the HDMI only seems to work if the ethernet cable is plugged in. If I remove the ethernet cable the monitor goes black. I plug the cable back in - monitor works again. Every time.
Only the HDMI seems to be affected. Everything else works fine. Wifi adapter, SSH connection - all good. Just the HDMI/monitor somehow is affected by the ethernet being plugged in or out.
I tried powering the Pi with a very capable power adapter as well as an active USB hub. Doesn't make any difference.
What's going on here?