My early-2008 15" Macbook Pro (see signature for specs) has recently started beachballing and becomeing unresponsive exactly one hour after the machine has been switched on. It does this every single time, so the machine can no longer be used for more than an hour. I have not changed the spec of the machine for more than 3 years, but this issue has only started occurring this year. I have had the machine looked at my our local Apple Reseller, who did a hardware test, but they could find nothing wrong with it. I've also wiped the disc and done a fresh install of OS X 10.9, but still get the same problem. It's very strange! NB: the main motherboard was replaced (for free) about 3 years ago (due to something faulty with the graphics) and the keyboard, battery and harddisc have all been replaced (the latter with an SSD), as was the RAM (up to 6Gb), so many of the components are not as old as 2008! Does anyone have any idea what might be wrong, or experienced any similar problems?