Today I awoke to the sound of the CPU temp warning beeps from my PC. The CPU has not overheated since I replaced the stock heat sink with an oversized copper core heat sink tower over a year ago. The CPU usually runs at abut 120F but is triggering the alarm at 145F. The fan on the heatsink is working although the bios shows no RPM speed for the fan (I'm not sure it ever showed this). I've changed nothing to cause this problem - the environment is not hot, no CPU intensive applications running, the heatsink feels cool but the CPU area does feel hot, plenty of case fans, with no other signs of excessive heat. Can the thermal paste break-down over time? Because of space limitations it was next to impossible to install the heat sink tower - getting it to snap into place was very difficult and I don't look forward to doing it again to replace the thermal paste unless I have to (if that's necessary). It is unusable right now due to the fast overheating. Is there a simple reason for this that I'm overlooking?