Remove the HS/F, scrape off the thermal pad with a plastic (not metal) tool, clean the heatsink and CPU core thoroughly with alcohol, and remount it with a premium compound like Arctic Silver or Antec Preference. Preparation and application instructions are at
www.arcticsilver.com .
I don't know why it gets unstable at such a low temp - P3's should go to at least 60 without even breaking a sweat. I wonder if the Via chipset is causing instabilities?