Freezes, reset & power switch do nothing

[simb]

Biostar TA790GXB A2+
AMD X4 940 3.0
GSkill F2-8500CL5D

I've seen this happen 3 times in past week, system is idle, screen goes black & says no signal. Disk activity light is on, solid, don't hear any disk activity. The reset button or power button or ctrl-alt-delete will do nothing. I have to shut the power off on the PS, turn it back on then system will restart ok. Build is 3 years old & has been problem free till now.

 

[simb]

PSU is SIGMA SHARK SP-585

memtest runs ok
burnintest runs ok

Nothing new had been recently added. I've had it do a repeat performance twice more this morning

 

[simb]

Replaced power supply & rather disappointed to see the same problem this morning.

 

[JMer806]

Check the connections from your case to your MOBO. It sounds like none of them are connected. I'm not sure about your video troubles, though. I assume from your specs that you aren't using a discrete GPU?

 

[simb]

I guess I was unclear. PC works normally, case switches work normally. Then, at some unpredictable time, screen goes black, pc is frozen, fans still running and at that point the case power & reset switches don't work. I have to power cycle at (now new) power supply & pc will start up normally, for a while. There is nothing relevant in the event viewer, temps seem normal. I'm thinking now it must be the mobo.