PeteA,
The cable I bought was a Sandstrom Silver series 2m 21Gbps. Got a superb deal for it at my local PC World as it was the last one they had in stock. It's connected to a Samsung UE40JU6740 which has 4 HDMI ports. I have tried all the ports. Each can be configured for various devices - not sure what this does however (it's not described in the user guide) - but I have tried them all. I have also had HDMI-CEC on and off on the TV and the T8 Mini.
The outcome is I can only get the T8 to connect at 1080p or 720p @ 60mhz.
I have spoken to Samsung and they diagnosed it an issue with the initial handshake between the TV and the T8. It's at this point where the device T8 decides what it is connected to via HDMI. As this TV works with the Samsung recently announced 4k BluRay, which I have seen connected to the same model as mine in the shop, then my conclusion is the fault lies with the T8 mini. So no matter which cable I have tried it made no difference in my case.
This does not help you however with your issue I'm afraid.
I just hope Droidbox can come up with a resolution for both of us.
Has anybody else got a similar issue?