Hi Robert,
In order to run Bootable Diagnostics at a minimum the display must be capable of a 1024x768 resolution. Is there any way to connect another display that is capable of a higher resolution, or are you limited by the video card or configuration?
There are only a couple of immediate solutions that I can suggest if a separate display doesn't work:
1. Running Service Center for DOS on the system instead (less than ideal, but it does provide a capable alternate testing environment).
2. Modify the boot command from the selection screen during boot up (either by pressing "Tab" or the "e" key for legacy or UEFI respectively), Replace the "quiet splash" option with "video=1024x768@60" and press 'Enter'.
Option 2 may not work with a display that is locked at 800x600, but I would still recommend giving it a try anyway.
Please let me know the results of your attempts and if you have any other questions or issues.