I have a 2009 iMac that has been working fine (but slowly) until now. I decided to upgrade the hard drive to a 500 GB Intel SSD (MZ-7TD500BW) and put in 16 GB of RAM (PNY Optima PC3-8500). I've performed a fresh installation of Mountain Lion and run memtest on the RAM. Everything passed great. I also repaired the file system permissions with Disk Utility.
But every once in a while the screen stops updating. When this happens, the mouse cursor still moves and everything seems to be functioning normally. For example, if a youtube video was playing, it continues to do so. But the screen freezes.
Has anyone seen this before, and if so, do you know a solution? So far we've been having to do a hard restart.