random reboots without BSOD - sometimes several daily

[tgfyhre]

For around 2 months now my PC has been shutting down unexpectedly and rebooting at random times. Sometimes at idle, sometimes while downloading, sometimes while browsing. Several weeks ago it was bad - it went through several reboots before coming back up. I ran memtest86+ after that, from the CD, and it rebooted twice during the memtest - off the disc - but then memtest ran fine after that. I then made the mistake of scheduling a checkdsk. In the middle of the checkdsk it rebooted again, corrupting something, and Windows would no longer load. I thought I had lost everything. Thankfully I used the Windows 7 32 bit install disk to attempt a repair, which failed at first, but then managed to repair the system by using a restore point.

For a few weeks everything seemed okay after that - no new crashes. Now for the last 2 days I've had several reboots again, just like before. I have no idea what is causing it. I get no BSOD - it just instantly starts to reboot with no warning. I ran WhoCrashed and it said:

Crash dumps are enabled on your computer.
No valid crash dumps have been found on your computer.

One thing I've been noticing in the event viewer is a ton of Microsoft-Windows-SharedAccess_NAT events. The events say:

"The DNS proxy agent was unable to allocate 0 bytes of memory. This may indicate that the system is low on virtual memory, or that the memory manager has encountered an internal error."

I'm including some logs from the SF_Diagnostic_Tool. You'll see lots of the Microsoft-Windows-SharedAccess_NAT errors on the EventSys log I included. I have no idea if this is related but I thought I should mention it. Can anybody please help me try to figure this out? I'm really worried about this. I built my custom Windows 7 PC around 3 years ago and it's been going strong until these reboots started to happen recently. The fact that it rebooted several weeks ago while running memtest86+ off a CD makes me think it can't be Windows related. But I'm stumped. My PC was fine for the last few weeks - now it's rebooting again.

My last crash/reboot was just a couple hours ago. The EventSys log says:

The previous system shutdown at 11:02:58 PM on ?4/?17/?2014 was unexpected.

 

[Gandalf]

Suggest you give this a try...
Use the System File Checker tool to repair missing or corrupted system files