XPS 8900 boot-up problem

For several weeks I have had a boot-up problem with my Dell XPS 8900. I have Windows 10 Home with all updates installed, and all drivers are up to date, according to Support Assist.

The symptoms are that I power on and the Dell logo appears with the spinning wheel underneath, then nothing more happens although the power on button has the white LED showing. The screen is black. If I press the power button and hold it in to turn off the PC, then restart the PC it fires up correctly.

In an attempt to cure the problem I have found a couple of tips on the internet but neither has worked. These are to change UEFI Boot Mode to Secure Boot ON. The other is to Disable C state in the BIOS and I have also disabled sleep. All of these changes are still in operation.

I have disconnected USB peripherals one by one and that has not changed matters.

A diagnostic test shows that the NVIDIA GeForce GTX 960 fails the Primary Surface Test.

The computer restarts fine - the trouble only shows when starting from cold.

Any help would be welcome.

Thanks.

Mike