Very strange blank screen behaviour at boot time and beyond.with AMD A8 CPU

I have a desktop computer running an AMD A8-5600K CPU and it is frequently showing a black blank screen at boot up. I can boot the PC from power on but I do not see any BIOS / UEFI messages. (I can't enter BIOS or UEFI screens either.) Occasionally the first thing to come up on the screen is the windows load screen, but 9 times out of time NOTHING EVER comes up on the screen!  I know the PC is starting up and loading windows because once it's up I can remote into it using Teamviewer (and then I discover it seems to be locked into a screen resolution of 640 x 480 !!).
I have tried new cables, VGA, and DVI to two different screens - none of them work. (But I know the cables and screens work with other PCs.)  I updated the Windows 10 drivers for the Radeon HD APU (which made no difference and separately I tried it with an nVidia graphics card but none of these actions made any difference.  I don't really think this is a Windows problem because even with hard disks disconnected and after removing and replacing the motherboard CMOS battery I still get no BIOS/ UEFI screens or messages. On the one time in 10 that it did work it worked perfectly at usual 1920 x 1080 resolution. I was able to restart it from there and it came back OK but after a shutdown it was back to blank screen . Exasperated !