Build ran fine for two weeks. Then ran Prime95 to stress test 4Ghz OC and core temp hit 87C before I could switch off the system powered off. Restarted system and reduced to 3.8Ghz and ran Prime95 again and system hit 83C and the system powered off.

Now, the fans spin up briefly, the PHASE LED lights on the motherboard all flash on, and then it goes quiet. This will repeat every 5 seconds or so, but doesn't get any further. No beeps, no other indications.

In troubleshooting so far, I have removed all components in turn including the CPU with just the PSU, 24pin ATX connected and the mobo. Reset the CMOS. The same thing happens Phase LED lights flash on repeatedly ever 5 seconds or so.

Any ideas pls?

GA-P55A-UD3 mobo
Corair TX650W PSU
I5 750 CPU
XFX 1GB XT 5870 Graphics Card