I recently bought a new Onkyo TXSR309 and I'm having issues switching between HDMI inputs, I'm hoping its a setting and not a fault.

I have a media PC and my virgin V+ box plugged into the HDMI in's, I have an LG tv plugged into the HDMI out.

When I switch to the PC input it functions fine but when I switch back to the V+ input there is no sound and a black screen, the image occasionally flashes onto the screen for a millisecond. The HDMI logo on the box flashes randomly. Turning the amp on and off resolves this and the V+ video and sound displays fine.

Anyone got any ideas?

I used to have an issue where my TV wouldn't switch down in resolution via a switch box.
Set your V+ box and PC to 720p and then see if you have the problem. 

Yep  Tried a selection of different resolutions, double checked all the connections, tried manually switching (buttons instead of remote) and nothing helps. Annoyingly its an intermittent issue.

Im really hoping BestBuy have a good return procedure, I cant really get to the store and the thing weights a tonne.

We shall see I suppose!

Bit of a long shot, but have you tried a different cable? if you're using a long HDMI run and the cable isn't quite up to it I think this can cause issues. It's worth trying a different/shorter cable anyway just to eliminate it from the equation.

Good call, I am using some cheap ebay 10m HDMI cable. I'll give it a go.

Cheap cables are fine for short lengths, but even HDMI cables require a certain quality standard above about 10m. I got one from http://www.thatcable.com/ that works a treat.

Switching cables did nothing but I switched inputs; moves the V+ box to BD/DVD input and moved the HTPC to CBL/SAT and its worked ever since.

I think it must be a fault on the box but as its working now i cant be bothered to send it back.

Cheers for the assistance.

Good luck, the Onkyo stuff is fantastic