Yeah, I'll second what's already been said. Check the TIM, check the HSF mounting bracket (I've had those break off) and make sure the fan you installed is actually spinning.
I had a GX280 whose fan failed - the computer chugged along for I-don't-know-how-long before anybody did anything intensive enough to cause a thermal event failure. (It took a couple weeks to get the replacement part in, folks were still using the machine, and it didn't happen a second time. The PSU must have provided "enough" airflow.)
Dell really kinda over-engineered those for cool'n'quiet running.
Scientific progress goes "Boink?"
Windows Gaming Box: AsRock Z77E-ITX / i5-3570k @ 4GHz / 8GB RAM / GTX 660Ti / Samsung EVO 250GB / Seagate Momentus 750GB / Silverstone SG05.
FreeBSD Server: SuperMicro X10SLM-F / Pentium G3250 / 16GB ECC DDR3 / 4x Toshiba DT01ACA200, 3x DT01ACA300, 1x DT01ABA200 / Fractal Node 804