Wow that must be frustrating. Have you tried disconnecting the HDMi cable from both devices and turning it on to see if it is some kind of RF interference. Then try a different HDMI cable possibly.

Need to try and establish where the problem is originating...

Just signed up on here today in a fit of frustration having experienced similar issues to you! Not sure if you still have the problem but for what its worth......

I have a Sony Bravia KDL-40HX803 TV with Sony HDD Recorder HXD995 and Blu Ray Player BDPS480 both connected via HDMI (also Xbox 360 via HDMI, Original Xbox via Scart and Sony Camcorder via HDMI).

All was working fine before today - I could have the TV on, the Blu Ray on and the HDD recorder on, all at the same time, switch between all of them and see all channels including HD channels etc as expected with all units running.

I then decided this morning to do a tidy up of the rats nest of cables at the back and label everything, so switched it all off, disconnected everything, labelled it all up and reconnected everything just as it was before.

Only difference now was that when I had the TV on and then switched either the Blu Ray or HDD recorder on, all BBC channels apart from the BBC HD channels suddenly lost the signal. No signal at all. Turn off the Blu ray and HDD Recorder and the signal comes back!! I was baffled to say the least.

I updated the firmware on the HDD recorder, as the Sony site suggested that may help - but no difference. Retuned all channels on TV and HDD recorder box but again no use.

I then refitted the TV aerial plug on to the external RF cable as it was suggested that may help on various other websites. It seemed to help a little as it started to get a scrambled signal this time when the boxes where switched on, rather than no signal at all.

I then powered the whole lot off down to power plug level and the started it all up again - TV, then HDD recorder, then Blu Ray. Signal slowly improved on the BBC channels and after an hour or so of leaving all boxes on everything seemed to have settled and be working again.

I read internet postings of HDMI cables and RF cables sending out strong signals which interfere with each other so I tried to pull the cables away from each other as much as possible which may have helped.

Don't know if any of that will help you or anyone else but the symptoms sounded similar and equally annoying so hopefully it may give you a couple of thigs to try.

PS - I was under time pressure also as my wife was going mental that I had knackered the telly and Downton Abbey was scheduled to record tonight....phew ;-)

Good Luck