Crazy Temperature Change After PSU & GPU Upgrade

Hi there. I have been working on my first build, a modest single core system.

Here's the specs:

Foxconn 661mxplus, socket 478
Pentium 4 PRESCOTT 3.2Ghz 1M/800fsb
2Gb RAM
Antec 550 Watt PSU
NVIDIA GeForce FX5500

This whole project started with a Dell Dimension 2400 with a bad MoBo. I got the Foxconn board, and decided to upgrade to the 3.2Ghz P4 at the same time. I cleaned it off really well, and applied a rice grain sized glob of Arctic Silver and spread it out smoothly using a latex gloved finger. Got everything fired up, and checked out the cpu temp in BIOS. It was reading somewhere in the low 30's after being on for an hour or so without much load. Then I decided to add the video card and replace the crappy Dell 200 watt PSU, so I just got the Antec. I installed everything and turned it on yesterday. After being on for a little while, and not doing anything too intense, i decided to check the CPU temp again when I was rebooting. This time it was reading 60 degrees Celsius! I shut it down for a hour, then turned it back on and checked again. It started out at 50, then quickly climbed back up to 60. I downloaded SiSoft Sandra, and checked it with that, and it was giving different results. Sandra said it was averaging 30-40, but every 5 seconds when it rechecked, the temp reading would fluctuate drastically : sometimes as low as 18, but never higher than 55. CPU fan speed is pretty stable at around 2280 RPM. I should also mention that the side of the case is open, and the ambient temp of the room has been right around 25 C.

So, why the drastic difference between the BIOS reading and Sandra? Which one should I go by?

Is there any way that changing the PSU and adding a video card would screw with the sensors' accuracy, or actually raise the temp that much? I should also mention that I just fired the computer up after being off all night, and the immediate temp that the BIOS gave me was 43 degrees Celsius, which has climbed back up to 60 over the last half hour. Its still on the cooler side of 25 in the room, so I don't know how it could climb up that high that fast only seconds after starting up.

I guess I could try re-setting the heatsink, but i feel like i did a pretty good job the first time. Anyone have any insight on this? Thanks!