Hey guys,
I've got three Iphone 3g 8gb sitting in front of me that all need their minor repairs. I need to replace the main board in one, so I decided to use one of the boards from the others.

Here's where the problem starts. I've tried both of the other boards, and they both either show the apple sign on start-up and freeze, or they go to the lock screen and freeze. I can put the original board back into the phone, and it works perfectly.

What am I doing wrong here?