iphone 3gs will not boot past apple logo

[Jade1]

My 3gs iphone had a cracked screen but otherwise worked fine. had recently upgraded to iOs5 and all was good. i eventually went and got the screen fixed and worked fine until a few hours later it froze and when i reset it didnt go past apple. tried again and no joy so put into DFU mode and restored but still not oging past apple logo.
Thought a 3G screen was put on so went back and had screen replaced and phone worked fine.. for 2 hours. Back to apple screen. please help i need it back working!!!