WardMan

Thanks. It's routed exactly as you've outlined. When it does cut out, I try the tuner on the HD500X and there's no signal so it does look like it's the VDI socket. I'll have a good look at it tomorrow and see if it's dirty.
 
I didn't get the USB dongle with the Variax, but I have a spare network cable I'll try out just to eliminate the possibility of it being the VDI cable.
 
Thanks for your suggestion.