Mac shuts down when I move it.

I have been having a problem recently where when I move my MacBook (old White 2,1) and especially when I unplug it. I have rEFIt installed and I think the problem started when I installed MacPorts (something which promised to do what I want and then promptly didn't). This created an /opt directory which I deleted, along with everything thing else it told me to get rid of to uninstall. I have also uninstalled rEFIt temporarily for troubleshooting purposes.

I think this is a software problem because it didn't happen when booted into my Linux LiveCD and I can't think of any time I might have dropped it. However sometimes, when it goes to sleep it might refuse to wake up and even a good press of the power button won't turn it on. I just have to wait a minuite or two and then try.

I have had a similar problem before and a reinstall of OS X fixed it (or so I thought) but I'm not in the mood for reinstalling now, what with exam season and all.

Thanks.