crisis,
If nothing else seems to work, let's do a complete Apple Hardware Test.
When you purchased your Mac, the box should have included a CD, DVD, or USB Software Reinstall Drive with an application called "Apple Hardware Test" on it. Depending on when you bought your computer, this could be an independent disc, or it may be included on the Mac OS X Install Disc.
Apple Hardware Test can run only when you start up from the disc or USB drive it came on; don’t bother trying to copy it to your hard disk. This program performs a series of diagnostic tests on your Mac’s hardware, including the AirPort card, logic board, hard drive, RAM, and video RAM. It doesn’t repair anything, and it doesn’t look for problems such as directory errors that are the province of Disk Utility.
To run Apple Hardware Test, follow these steps:
1. Insert the disc or USB drive with Apple Hardware Test on it into your Mac and restart, holding down the Option key until icons appear representing the available startup volumes.
Note: If the disc label specifies holding down a key other than Option, do what the label says.
2. Click the Apple Hardware Test icon, and then click the right arrow.
3. After the program loads, select a language and click the right arrow.
4. On the Hardware Tests tab, click Extended Test.
5. Go make yourself a sandwitch and watch a movie or enjoy a stroll around your neighborhood. This test takes a while!
The screen may say something like “Estimated time: 10–15 minutes, or longer depending on the amount of memory installed.” Take the “or longer” part seriously. On an old MacBook with only 2 GB of RAM, the test takes almost an hour and 45 minutes.
6. If all is well, the word “Passed” appears next to all the applicable tests in the Test Results area. If not, a failure message appears; if this happens, we'll go to the next step.
7. Click Restart to restart your computer.
Steve391