Download this app:
http://www.sisoftware.demon.co.uk/sa...html/dload.htm
After you install it, run the burn-in test. You can select it to run only the memory, or CPU, HDD, or all. Run it for a few hours (I usually do it overnight). If it doesn't lock up, it's pretty stable.
If it does lock up either;
A)Increase the voltage going to your memory
B)Increase the timings (HIGHER, NOT LOWER)