It's become extremely slow to load and run programs, even simple functions like getting iTunes or Chrome booted up can take 10 minutes.
I also have regular (once a day) crashes of iTunes, Aperture, iMovie, Chrome etc.
Regular (every few days) unprompted reboots, 20-30 minutes to get it to wake up from the Sleep function overnight.
I recently went through the dreaded (I don't know if it's really dreaded, but it sounded good) MacOS update crash bug, where the iMac wouldn't load past a black screen. Finally figured out it was a known issue and followed the steps I found online to get the update to fully load, but I am still having the above issues.
Try booting from an external drive and see if the problems continue?