Replacement Of Thermal Paste Causes CPU Load To Fluctuate Crazy

When I first tested my i7 920 system ( with Vista Ultimate 64 bit OS ) for video encoding, I was able to covert a 700 Mb AVI file to DVD using Nero in only 14 minutes. This was almost 4 times faster than my Athlon X2 64-bit 6000+ 3.0Ghz.

Before conducting any Over-clocking tests, I decided to replace the stock thermal compound with OCZ Extreme Freeze.

After reinstalling the heat sink, I ran the same video encoding test as above and it took nearly 40 minutes to perform the identical task.

What’s odd is the CPU load starts off at about 70% and slowly declines to 30%. So as Nero runs, the estimated time to complete the task on the application window would gradually increase.

Additionally, I did a fresh install of Vista Ultimate 64 as well as XP pro 32bit and XP Pro 64bit on another HHD and the same thing occurred. The CPU usage would start off at 70% and decrease to 30%.

I even reverted the BIOS to default settings and this made no difference as well nor did assigning "High Priority" for Nero in the task manager.

I don't believe this has to overheating because the CPU temp never approached the threshold of 60degC and this is monitored by 2 different programs (Speed Fan and Asus Probe II). Furthermore, CPU idling temp is in the mid 30's.

Anyone have an idea of what caused this.

PS

I have not Overclocked my system at all prior to this.