In spite of the AHT results it does not sound like software or system related issues. If I were to name my "usual suspects" in a case like yours the list would include in rough order:
  1. RAM
  2. CPU
  3. Logic Board
  4. environmental issues (ambient temperature, local air circulation, air circulation around the MacBook, etc.)
Since you are dealing with an early 2009 model, it sounds to me like time to take your MacBook to the Genius Desk and let them run their diagnostics which are more comprehensive than yours. FWIW I have always found Marcel Bresink's Hardware Monitor to be very reliable in checking internal system temperatures, power levels, fan speeds, etc.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein