Thanks, I saw this page and called a local apple service center. They said that they would need to have my machine for a while to run a diagnostic test, but if it turned out not to be the GPU that I would be on the hook for a $47.50 service charge. It it was the GPU, the apple would pick up the bill and the repair charges. I'm trying to get an idea of how likely my machine has a faulty GPU before leaving it with them. In addition to the machine becoming unresponsive and a pattern of big green pixels appearing, it develops some additional symptoms:
- seems to get hot with the fans going loud
- it takes many tries to restart
- while trying to restart, green pattern of pixels shows up in the apple logo and the spinning indicator (when this happens, the machine does not start up successfully, just keeps trying)
- during on of the "freezes" with the pattern of big green pixels, it appeared to continue to copy files (started to copy a big directory from on external disk to another) and did a time machine backup
- when finally successful in starting up again, iStat Pro showed that the GPU was running at about 70 C
SL