I bought my macbook in July '06. About 1 month ago my computer had a problem and had a new logicboard put in. After about 1 week of using it my Macbook OS X (10.4.11) is shutting down by itself, restarting itself sometimes, and when I start it up the screen flicks black for a second. This happens within around 2 hours of using it. I keep the battery charger plugged. What could be wrong? I tried the PRAM reset.