My iPhone only works on iOS 3.1.3. Any ideas how to make it work on iOS 4.2.1?

Hi guys

I just bought an iPhone 3G very cheap, for 20$. Its battery puffed up and pushed up the screen a little bit, and also it broke about 3 screws on the right side, so the vibrator is not working in it. By the way, i bought a new battery, and everything was fine.( The screen has no errors, the phone functionality is 100%, the buttons are fine.) i noticed, that the phone restarts, or just switches off in 5 minutes. I restored it, (iOS4.2.1) setting as new iPhone and it continued to restart. So I tried install an iOS 3.1.3 on it, and now it's working, but I'm wondering why is this happening, and how can I install iOS 4.2.1 on it without these restarts.

Thanks for help