This is a known problem with the E6420 14" laptop and Quad core CPU's. I have a E6420 and E6520. The E6520 handles the heat much better and tops out at 85C GPU and 95C CPU when running Diablo 3 for an hour on max resolution and max graphic settings. My laptop shipped with the single heatpipe heatsink, it would downclock CPU because of excessive heat and even shutdown. After I received the Dual pipe heatsink with a new thermal pad, my temps dropped to 100C max on CPU and GPU and laptop no longer shuts down due to overheating.
The problem is Dell packed a lot of power into a 14" laptop. When running at max load, the cooling system simply can't keep up. Dell should really fix this but you can do a few improvements on your end. FYI - Do not attempt if you are not comfortable doing this or are tech / mechanical savvy.
Here is what I recommend:
1. Ensure you have the latest BIOS installed. Dell has changed the thermal curves for the fan in recent bios updates.
2. Ensure you have the DUAL HEATPIPE heatsink DELL P/N - TYP01. My E6420 shipped with the wrong Single pipe heatsink pictured below.
Single pipe heatsink - ONLY FOR DUAL CORE i5, i7. NOT FOR i7 QM QUAD CORES.
There is a "gap" between the heatsink and the Nvidia NVS 4200M GPU. Dell uses a <ADMIN NOTE: Profanity removed as per TOU> thermal pad to transfer heat. a lot of users have done the following to reduce temps even lower by replacing the thermal pad with a copper shim sandwiched between two layers of thermal paste / compound. I'm sure Dell probably does not support this below mod even though users have reported lower temps after the mod which in turn stresses components less.
3. Order this copper shim kit. www.ebay.com/.../260718120394
4. Use a good thermal paste / compound such as Céramique 2: www.newegg.com/.../Product.aspx
Lastly - search through the forums on this website as there is alot of information about your E6420.
i7-2760QM turbo up to 3.5ghz
16GB DDR3 @1600MHZ - yes the E6420 and E6520 will accept 16GB and run at 1600MHZ if you buy 1600Mhz Ram AND have a Quad core CPU that Supports 1600Mhz ram natively. search for your cpu here and look under "memory types" for supported speeds. "Max memory size" for max memory your CPU supports.
500GB HDD @ 7200RPM
1600 x 900 HD Screen w webcam
Intel 6250 Wifi