I have a late 2012 iMac. Lately it started to act up: beach ball appears all the time to the point that the computer is unusable, even when all I do is online surfing. I thought it may be a software issue, and I re-installed Yosemite clean. I still get the beach ball. I verified the disk with disk utilities and I get no errors. Does anyone have any idea what may be wrong or how to debug?