This last week, on two occasions -- i.e. not consistently -- when I try to boot up my MacBook Pro with SSD, I get a whirling circle symbol below the gray Apple logo. I have to re-start several times, and then it eventually goes to the desktop. I assume this is telling me something could be about to go very wrong, and it's time to take action rather than wait till it really fails. I wonder if it's a hardware or software thing? Maybe time to re-install the system to get everything cleaned out? Mac is about 2 years old. Suggestions?