When I connect a second screen to my laptop, via DisplayPort-DisplayPort I get an unusable desktop.  The mouse cursor on the second screen is fine, but everything is garbled (weird blocky behaviour, and the actual desktop image is completely unintelligible).  Of I disable switchable graphics, then both screens go the same way.

I've done a boot-time diagnostics, and that completed successfully.

I also tried a diagnostic on the FirePro from the Dell Support Centre (pc doctor) application and that failed on one or two of the tests.

I am running Windows 10 x64, and have installed the Dell 'official' Intel Drivers, followed by the AMD drivers - no joy.

I have had it work on two separate occasions.  But now it doesn't work at all.

Appreciate any help.