windows 10 1809 is starting/stopping

HP18-5021 computer
HDD was configured as GPT. I reconfigured it to MBR and clean installed W10 V1809 and updated. I installed a few generic programs and do not believe any are responsible for this problem.
When w10 starts to boot, the progress circles run smoothly. As soon as it gets to the logon screen, the circles begin to start/stop about every 1 sec for about 1/2 sec. When it is fully booted, the start/stop continues and everything is affected - mouse, programs, etc.
Process explorer is showing SYSTEM with very high usage, like 25-35%. This isn't normal. Another computer shows <1% usage.
DPCs are constantly green <500µs.
Restart in diagnostic mode - same issue.
What is causing this?

obviously, I don't know much about GPT. Every encounter I have had with it so far has not been positive.
I converted because I wanted to clean the disk up - it had a number of orphaned patitions. Once I got them all deleted with EaseUS I tried to install Windows and it said it couldn't on a GPT disk. So I changed it.
I ran chkdsk /f and dism and it said there were problems - couldn't find event handle and the log said it couldn't find oem(x) files.
But I had no idea why not.
running chkdsk /R now and it seems to be taking a lot longer so maybe it is fixing something.

winver is as you stated

Can you use another computer and go to the manufacturer's website of the company that made your hard drive? Download their disk checker from their site. Run it and make sure your hard drive is not failing. The test will tell you if it is good or not. Then go from there depending on what you find out about the drive.

there are no errors showing for chkdsk except the "Unable to create handle for event log"
Dism however has plenty of errors in the log.Attachment 232228

any ideas?

I converted the GPT to MBR using diskpart. Did I miss something?

1) Open administrative command prompt and type or copy and paste:
2) sfc /scannow
3) dism /online /cleanup-image /restorehealth
4) chkdsk /scan
5) wmic recoveros set autoreboot = false
6) When these have completed > right click on the top bar or title bar of the administrative command prompt box > left click on edit then select all > right click on the top bar again > left click on edit then copy > paste into the thread

@zbook
OF COURSE - every strange problem has to be associated with the VIDEO driver. How can I forget that?
NOTHING shows bad in any of those 6 steps. I dug deeper into process explorer and the high CPU usage under System and found it was atikmdag.sys (from an ATI driver I installed) using all the CPU resources.
I deleted the Display driver and files. Rebooted. Windows installed another driver and I updated that via the device manager.

All OK now.
thanks
@Ken - thanks - I will review that just to make sure - I did do a clean and I think it is good. I try to always do a CLEAN INSTALL and have done literally 100s so that is good. This is not on my P8Z77 it is an HP18. Don't know why it did not want to install on GPT. I guess I need to play with that and figure GPT out since it is THE thing, right? The USB I have is configured for both BIOS and UEFI. The system was set for UEFI. I had to change the protected boot to set legacy devices and get it to boot from the USB.

don't know about that. I will check it. But the USB booted up and started to install windows, then gave me the message.