What kind of thermal grease/paste are you using on these chips when you apply/re-apply the heatsink? Try going to something like Artic Silver and lay on just enough to create a thin film over the CPU die. It's only supposed to fill in the small air gaps between the two metal surfaces, not do all of the heat transfer. A utility knife blade works great for spreading it on thin & even.
Also make sure you're placing the heat sink on squarely & securely the first time. Remember socket A heatsinks are offset to one side, and the spring, clip, etc, needs to be putting most of the pressure directly over the CPU die. The die on your first chip may have been cracked by uneven pressure during installation (I've done it myself) or when/if the clip came loose from one end.
**Rubbing alcohol & Q-tips are great for cleaning heatsinks & CPU dies between applications**
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