Power Supply Question

I recently built a new system using a AMD Phenom x4 with an XFX motherboard. The case came with a measly 400w power supply which seems to be working fine. The motherboard has the 8 pin power connector for the beefier processor, but for now I've been plugging in the 4 pin since my weak power supply doesn't have an 8 pin. Here's the problem, I upgraded to a 1000w power supply with an 8 pin connector and the system frequently just shuts off. It will stay booted up maybe 15-20 minutes then just die. It will start back up no problem, so I'm not sure what's going on. Possible grounding issue? I've since reverted back to the 400w since it is much more stable. The 1000w is running fine on an older Dell spare pc I have laying around, so I'm not sure what's causing it to shut off on my newer system.

Any ideas?

Thanks!

The 24 pin has all 24 there. The odd thing is the 1000w is working fine on this older Dell. The only difference is the Dell has a 4 pin for the CPU where as my newer PC has the 8 pin for the AMD Phenom x4. The 1000w supply is an MSI Turbostream. The 400w is some generic brand that came with the case.

MSI is a Tier 5 power supply: not recommended.
I would return it and get a better one.

Oh snap!!! Let's cut and paste some Tom's articles. So this is what I found out. I downloaded freeware utility - SpeedFan - and discovered the obvious - the 8 pin connector supplied more juice to the Phenom x4 than the 4 pin. The CPU was overheating and shutting down the PC. I purchased a quality CPU fan and the smokin' PC works like a champ, even with the *cough* tier 5 power supply.

TY, I guess...