Help - PC turns off randomly and does not restart for 10 mins

[geejay]

I bought the following system in Feb '11.

Processor: Intel Core i3 540
Mobo: Intel DH55TC (BIOS ver 0037)
RAM: 2x2GB DDR3 1333 Kingston
Cabinet: i-ball piano
SMPS: Built-in SMPS not sure whether it is 250 or 400 W

For the last couple of weeks, my PC has been randomly switching off. I am able to restart only after about 10 mins. Sometimes while booting itself the system shuts down.

I carried out the following troubleshooting steps, could not identify what the problem is..

1. Checked Processor fan, SMPS fan, and cabinet fan..all working fine.
2. Ran prime95 torture test for 12 hours..no errors..real temp showed processor temp of around 70 degC..surprising the system did not shutdown during this test.
3. Ran memtest86 - Both RAM sticks did not have any errors.
4. Took the PC to dealer from which I had purchased. The technician spent some 15mins and said there is no problem with the system.

I am at loss what could be causing this problem.

Seek inputs to identify and solve the problem.

[asingh]

^^
Highly probable it is the PSU.

[geejay]

Do you think the PSU has gone bad or undersized? FYI, there is no graphics card.

Further should I get a replacement or buy a new PSU?

[Gaurav Bhattacharjee]

Test the PSU on another system if possible. If it is the culprit then, yes, do get a new one.

Corsair CX400 @2.5k if you're willing to spend money on a solid PSU.