Chosen Solution

have you update the os or anything recently? the cocktail of troubleshooting i would try is

  1. boot into your recovery partition with Command-R on boot (assuming you are on Lion or later)
  2. select disk utility and select your Macintosh HD (or your boot partition) then under the first aid tab choose repair disk and let it run. then repair permissions and let that run :). the next thing i would try is to reset PRAM by hold Command+Option+P+R on boot after that reset SMC and steps to do that can be found here http://support.apple.com/kb/HT3964. let me know if any of those show any improvement to your symptoms :).