cpu - Dell Inspiron N5110 Overheats and Shuts Down from Past few Days

I have a Dell Inspiron N5110 with a 2.5Ghz i5-2450M processor, 4GB RAM and 1GB nVidia Graphics card running Windows 7 Home Premium, that's about 2 years old now.

When I previously used to play games like Skyrim: Elder Scrolls V and MS Flight Simulator X, the system would heat up a lot, but never used to automatically turn off. In recent days, when I play these same games (even at the lowest setting), the systems heats up, but automatically shuts down.

What can be causing this and how can this be stopped? Someone said I need to open up the system and put in new thermal paste. Is this the thing? Or could there be something else behind it.

I don't use a cooling pad, but there's sufficient ventilation. The cooling fan works fine and spins really hard. I don't have an Anti Virus program running as well, since this laptop isn't connected to the Internet. I updated all the drivers last week.

Edit

A friend had a similar problem with a similar Dell Studio model, and when I opened up the case, found a huge wad of dust/lint/fur blocking the fan's vent opening - much like a dryer lint filter. The clog wasn't visible until opening the case. Since you aren't using a cooling pad, odds are that the vacuum effect of the fan has been picking up stuff to clog it and now needs some cleaning. The thermal paste would not degrade in so short a time.