Laptop gaming problems

Hey, I have a problem with playing games, and recently I have started playing Minecraft. I launch and play minecraft smoothly with around 100-120 fps the first 5-10 min. Then it drops down a bit to 70 fps, but it is nothing unusual. But after 15-20 mins of playing the fps drops down to 1-14 fps and my pc gets really slow!!! After that, I have to close minecraft or any other game I try to play, and I have to restart my laptop. It also gets a bit hot. It does not happen when I am browsing for a long time, only when gaming. Please help if you can!

Info about my Laptop:

Model: Dell Studio XPS 1640

CPU: Intel Core Duo CPU T9600 2.80 GHz

RAM: 4 GB

OS: Windows 7 Ultimate 64-bit