My PowerBook was doing this last week. I think it had something to do with the Remote Desktop update, but I'm not sure. I reinstalled OS X, but that didn't take care of it. I resetted the power manager, reset the PRAM, nothing worked. I ended up reformatting and doing a clean install, and now everything works fine. I would recommend first resetting the power manager and resetting PRAM. If that doesn't work, do a reinstall. And if it comes down to it, as in my case, do a reformat. I hope you can solve your problem easily. Good luck.