|"The power supply has been tested and passed."|
We need the details about how you determined you think it's okay.
The only for sure test is to try another known good PS that has enough capacity and enough current available at +12v to handle your system with the HD 5770s installed
to try your power supply with another system that has very similar requirements.
It's probably, say, 100 times more likely the power supply is causing your problem, or many times more likely something else is causing your problem, than there's something wrong with your mboard. Mboards don't normally work fine for a long while then spontaneously start mis-behaving, unless they have the defective electroytic capacitor problem and even in that case that is extremely unlikely for a mboard that's less than about 3 years old. Many mboards these days no longer have any conventional electrolytic capacitors - they have "solid" capacitors.
If your PS can't actually meet the load requirements it is subjected to, it will often still work fine for a while, but it's running at as much as 100% of it's current or wattage capacity all the time or much of the time. PSs aren't designed to be continuously loaded over 80% or so. The PS will be slowly damaged over time, until it can't even meet it's previous maximums.
If you don't have this problem, then there's probably nothing wrong with your mboard....
Some mboards develop this problem - electrolytic capacitors were installed on them that were not properly made, and they fail eventually - the mboard manufacturer didn't know they were improperly made at the time the mboard was made.
Open up your case and examine the mboard to see if you have bad capacitors, and/or other findable signs of mboard damage .
This was the original bad capacitor problem - has some example pictures.
History of why the exploding capacitors and which mboard makers were affected:
What to look for, mboard symptoms, example pictures:
Home page that site
- what the problem is caused by
- he says there are STILL bad capacitors on more recent mboards.
Pictures of blown capacitors, other components, power supplies, Athlon cpu's, etc.: