What did I do wrong?

Intel PIII Slot1 550MHz @ 733MHz
256MB PC133 SDRAM

I have been running my PC at 733MHz with the HSF that came with it. I bought a Vantec P3-D5030 two days ago. I got rid of the original HSF and installed the new one yesterday. Motherboard Monitor 5 shows that my CPU temperature is lower than before (used to be around 33 deg when idle, now it is 28 deg). The problem is that the CPU is very temperature sensitive now. The temp jumps more than 10 degrees when I start an application like Word or Excel. When I play mp3 and video files the temperature rises up to 45 degrees. Then the computer randomly restarts. I was watching a DVD yesterday and the computer just restarted after about 20 minutes of viewing. I just ran Norton Antivirus to scan my HDD for viruses and MBM5 showed the temperature go up to 51 degrees. I shut Norton Antivirus down and the temp went down to 29. Then as I was typing this post the computer ramdomly restarted. What is happening? I have been running the CPU with original HSF from Intel for almost a year with any of the above problems. I appreciate any help in this matter.

Did that HSF come with a built in thermal pad? If so was it thick? Looks like your full load temps are high and causing instability.

Sounds like you don't have the HSF making proper contact with the cpu. Take it off and clean off the thermal compound from both the CPU and HSF. Get some thermal compound/grease and put a very thin coating on the CPU and then re-mount the HSF. Now take it off and look at where the cpu made contact to see if you are getting full contact.

I'm not familiar with your HSF but it could have been tilted, or there may have been a foreign body preventing contact, the thermal pad (if there was one) may have been too thick or just poor quality, the surface of the HSF may not be smooth - it may need lapping.