Phone shuts off every 15 minutes and turns on by itself

Last week I started a project to repair my brother's old broken iphone 3gs (no idea how it broke) mainly because I had the money, time, and the feel to explore my inner self (kind of corny I know). So I lent the phone to a local radio shack and asked what parts needed replacement and said the phone was in so much damage that it seemed that every part except for the camera, logic board, and dock connector seemed fine. So they guys at radio shack offered to repair it but the price didn't agree with me so I looked online for the best prices and trusted websites (advice from friends) and bought every part imaginable and started to repair. I successfully repaired the phone to it's former state but the only problem I encountered were that the power button would not work and the phone would randomly shut off and would turn itself back on. This would occur every 15 - 20 of use. A day later during work, there was a birthday party and some of the kids like to play games (a lot) and would play games on anything that has a screen, so then they found my phone in my room played around with it and left it on the couch where then my 350lb uncle came to have a sit and fortunately sat on the phone. a couple hours later when I came home after work I found my phone to be in good condition (on the outside) but then would shut down 5 seconds after plugging the phone into my computer to turn it on. So I open my phone to find that the contact between the battery and the logic board has been cleaned by rubbing alcohol and a fairly tiny piece on the dock connector fell out and seemed to have been ripped out by accident (my brother's way of trying to help). So now that pretty much sums up my problem, I know its a long story but I wanted to state myself clear to show what happened.