Random rebooting unknown reason

Computer spec:
CPU: Core i5 750
MB : Intel DP55WB
Mem: Crucial 1GB x2 DDR3
HDD: WD 500GB Black
PSU: Rosewill 400W
GPU: Asus 4350

I built a computer for my friend.
When I was building the computer, I did notice that the computer would randomly reboot during OS install. So we replaced the PSU from the stock 300W case PSU to the Rosewill.
Now he's complaining that it randomly reboots.
I told him to loosen some of the MB screws, still reboots.
He replaced the CPU fan with CoolerMaster Hyper 212+. Reboots, but it happens less.

So I brought the computer home and ran orthos to stress it and make it reboot.
Ran the SmallFFTS-Stress CPU for an hour, no error. Ran the Large in-place FFTs-stress some RAM for 20 minutes, no error. Ran "Blend-stress CPU and RAM" then it reboot after a couple minutes. After reboot, ran it again and it gave me an error saying the test failed after 5 minutes (but no reboot). Couldn't find stress.txt so can't tell you what it said. Ran the test again in blend and shows me the same message after a couple minutes.
So I thought, maybe it's the RAM.

Ran MEMTEST86+ for about half an hour, and it reboots again.

Did a google search and did find this topic that seems relevant:
http://www.tomshardware.com/forum/ [...] uggestions

There was no solution provided.

Does anyone have a clue to what's going on? Any help much appreciated folks

Related to that, I did notice that when it's been unplugged for a long time and I plug it in and press the power button, it won't start right away. It needs to take a few seconds to "charge" or something before i press the power button.

PSU related or MB?