D820 performance issues

[MatreX]

Hello all,

Have a strange issue that's boggling my mind here...

First, the setup:
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Intel Pentium D 820 with retail Intel HS/Fan
Foxconn 945G7MA-8ERKS2 mATX mobo
2GB DDR2-675 Corsair XMS
2x 160Gig Maxtor Sata/7200/8mb (in RAID Stripe)
LiteOn Dual Layer DVDRW
XFX GeForce 6800 GS (PCI-e)
SB X-Fi Music edition
Aspire Qpack case with Antec NEO HE 430 PSU
WinXP Pro (SP2)
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Okay, I can boot the pc and let it sit on the desktop idle and only have the task manager open to monitor cpu loads. Let 5-10min pass, and Core0 will start to show a load that will steadily make the graph rise until it's at 95-100% utilization. But, all this time, Core1shows the typical 0-3% idle utilization. Now, while Core0 is at ~100% and Core1 is at idle, my overall cpu utilization is at ~50-55%. If you look at the system processes while this is happening, it shows 95% system idle processes. The math doesnt add up. Also, system performance drops severly while this happens, and I can hear my cpu fan ramp up to compensate for the "mysterious" load. After about 20min, the Core0 graph will start to go down and performance returns.

Remedies attempted:

1)I've removed the Intel thermal pad, and installed Arctic Silver 5
1 Result) same problem exists

2) reloaded OS thinking I have a hosed install
2 Result) same problem exists

3) installed hotfix from MS for WinXP SP2 with DualCore
3 Result) same problem exists

4) updated BIOS to latest version
4 Result) same problem exists

Any help, or suggestions, are welcome and greatly appreciated.

Also, I'm very comfortable with trying most things as I've been a system builder for ~15yrs.