PC restarting randomly.

Hi, I'm posting this here because I believe the problem could be graphics card related.

For about a week now, my desktop PC has been randomly restarting (no BSOD or anything).
I have turned off the automatic restart setting in system settings, but it is still just restarting.
There also does not seem to be a minidump directory, so I can't check the logs.

I have made a couple of changes to my PC recently which could be the cause.
First, my PSU died about two weeks ago. I actually came home to the PC constantly turning itself off and attempting to restart every few seconds (it wasnt even booting up), and making a 'crackling' kind of sound.
Anyway, I got a new 630W PSU. (My old one was 650W).

Everything seemed to run fine after the new PSU went in.

Second, I recently bought a second monitor for a multi-monitor setup. However, since I also have my TV hooked up to my PC, I needed to buy a 2nd graphics card.
I think my problems started a couple of days after installing the new monitor and graphics card.
I can't seem to notice any pattern to when it restarts. It is not when the CPU or GPU is under load. I played games for a few hours with no problem.
But it has seemed to restart more frequently when viewing something on youtube (but not always).

Anyway, here are the system specs:
Windows 7
- Kuroutoshikou 630W PSU
- Asus P6T MB
- ATI HD 4850 (this is the card I have had for a couple of years and has been fine) *Both my monitors are plugged into this card.
- ATI HD 5450 by Gigabyte (this is the new card which I suspect may be causing the problem) *Only my TV is plugged into this card.


The HD5450 is installed in the 2nd SLI slot on the MB (should I try putting it in a regular PCI slot instead?)


I have been running HWMonitor also, but the temperatures don't seem to get particularly high.


I have actually just removed the HD5450 card to see if the problems persists, so I will post back with that too.


In the meantime, does anyone have any ideas about how I can go about diagnosing this problem?
Thanks!