I have an early 2008 Mac Pro that over the past months has been locking up (Freezing) on occasions.
Often after doing that upon restarting it starts to just a grey screen but leaving it for a day restarts ok but locks up after five minutes or so.
After experimenting what seems to fix it for a time is a Yosemite install (Refresh). After doing that it seems to work ok sometimes for weeks until acting up again.
Anyone have any ideas on whats causing this? If it was a hardware issue my thoughts are that it shouldn’t go away.