I recently installed an 89w Athlon 64 x2 5800+ along with the hk912 heatsink for 89w processors in my dell optiplex 740 mini tower. On boot up the fan gets a little fast and then goes back down to normal.The cpu temps are normal but the second I start up any program besides Mozilla (or other low cpu usage things) the fan starts to increase in speed rapidly until it goes full bore and without warning the computer shuts down. I have tried taking out the battery to reset the bios, enabling and disabling cool n quiet, cleaning the fan and, tightening the heatsink- all to no success. I have felt the heatsink and it is cool as can be.
The processor before this was the 65w athlon 64 x2 4800+ and I never had problems with it.
Sounds like the cpu is hitting it's high temp limit and shutting down before melting down. Download and install speed fan http://www.almico.com/speedfan.php then after booting, monitor the cpu temp when starting programs. It may be as simple as applying new thermal compound to you cpu/heatsink.
Just to confirm, you also have the shroud in place to direct airflow to/through the heatsink. If so, then I will suggest the following. Using AMuffin's guides here, http://www.tomshardware.com/forum/333968-28-thermal-pas...
replace the thermal compound on the HS and reseat it.
Hope it helps