With multiple, apparently unrelated potential hardware issues, I always suspect power too. Since you replaced your PSU, that rules the PSU out, but not necessarily power. I always recommend every computer be run off a "good" UPS with AVR. This is because a surge and spike protector is little more than a fancy and expensive extension cord. They do absolutely nothing for low voltage power anomalies like dips (opposite of spikes) and sags (opposite of surges) or brownouts (long duration sags). While these events are not necessarily damaging like excessive surges and spikes can be, they can and do result in unstable operation - freezes, sudden shutdowns and reboots.
Also, "dirty" power can come from an improperly wired or grounded wall outlet. So every home and every computer user should have access to a
AC Outlet Tester to ensure your outlet is properly wired
and grounded. I recommend one with a GFCI (ground fault circuit interrupt) indicator as it can be used to test bathroom and kitchen outlets (outlets near water) too. These testers can be found for your type and voltage outlet, foreign or domestic, (like
this one for the UK) at most home improvement stores, or even the electrical department at Wal-Mart. Use it to test all the outlets in the home and if a fault is shown, have it fixed by a qualified electrician.
I agree with softwaremaniac about BSODs. You might also check in Event viewer to see if you can see any errors just seconds before the crash occurs. No errors does not mean there are no errors - it can mean the crash was so sudden, there was no time for the OS to log the error.
You should reset all your clocks back the defaults while troubleshooting.
Also, there are several heat-sensitive components besides the CPU that can affect system stability too. So make sure the interior is clean of heat trapping dust and all case fans are spinning properly for a good front-to-back flow of cool air through the case. Consider blasting a desk fan into the open case if you suspect there may still be a heat issue.
You should probably test your RAM. I recommend
MemTest86. Allow the diagnostics to run for several passes or even overnight. You should have
no reported errors – not even one.
Note, however, that software-based RAM diagnostic tools are good, but none are conclusive. If they report any errors, even one, the RAM is bad. But it is not totally uncommon for them to report no problems, yet the RAM still fails in use, and/or when paired with other RAM. So, you might try running with just a single RAM stick to see if it fails. Repeat process with remaining modules, hopefully identifying the bad stick through a process of elimination. Just be sure to unplug the computer from the wall and touch bare metal of the case interior BEFORE reaching for the RAM to discharge any destructive static in your body.
At any rate, these are all things I suggest you check before spending any money on a new motherboard.
When you get a chance, please list your system specs, or fill out your system specs
here.