Computer freezes after starting and on re-boot says 'Warning!!! The previous overclocking had failed' - how to fix?

I bought a custom-built PC on Ebay a couple of years ago and for the most part it has worked fantastically, but I've been having a few problems with it recently. The system specs are as follows:

OS: Windows 10 Home
System manufacturer: MSI
Processor: Intel(R) Xeon(R)
CPU X5450
BIOS: American Megatrends Inc. V28.10
RAM: 6.00GB
Graphics card: Intel(R) G41 Express Chipset#

So, the problem I've been having is that when I go to start the computer, it typically freezes within a minute or two of startup. Sometimes it freezes before it even gets to the user desktop. I do a hard reboot, and sometimes it will restart the computer fine (but still freeze after a couple of minutes), but quite often the computer will make the whirring noise indicating it is trying to start, but then fail. After a couple of minutes of unsuccessful attempts, the restart will normally work again, and quite often I'll see a page saying "Warning!!! The previous overclocking had failed" (example). I don't really know what I'm doing so I haven't tried too many fixes, but I've tried reverting to fail-safe settings, reverting to optimised settings, and just keeping the settings as they are, but it doesn't seem to make too much of a difference - usually it will freeze after a few minutes either way.

However, after many unsuccessful attempts, the freezing suddenly stops and I can use the computer just fine (as I am doing now). When the issue first started happening it would only freeze a couple of times before working fine so it wasn't too much of an issue, but it's taking longer and longer now - this morning I was sat by the computer restarting it for two hours before it worked. The fact that it can work fine suggests to me that maybe there's no physical damage, but something is causing the problem nonetheless and I'd like to be able to fix it. I've made sure all my drivers are updated and I've tried disabling most of my startup processes, but neither of those attempts worked.

I've also had problems in the past with different BSoDs, mostly related to memory management I think but with lots of different error messages. I think it is probably related to the problem - over the past few weeks the cause of failure has been a pretty even split between freezing or BSoD, and either way I have to restart. Quite often the graphics will mess up when it freezes too, and lines will appear across the screen.

All advice is greatly appreciated and I'm happy to post any more information if needed - I'm not that computer-savvy though, so I might need to be guided a little bit. Thanks in advance