Intel Mac :: Keeps Crashing - Cleared Junk Files And Filters Are Up To Date

I've tried to troubleshoot this before. I've backed everything up on Time Machine, did an Erase/Install. Purchased MacKeeper. I restarted computer pressing shift key, then restarted computer to clear out any accumulated junk files. I've been running Virus Barrier for more than a year, and all filters are up to date. But it's getting worse! It used to happen once a day...today it happened 3 times. I thought it might be related to Chrome, but this morning, all I had open was Mail...

This is what happens: In whatever application I'm in, usually Chrome, after some action, sometimes just loading page, the rainbow time spinning wheel appears. Spotify stops playing music (though there is not window warning that I've gone offline or anything like that); I wait, wheel keeps going, I clicked on other open applications, they appear fine, go back to application with spinning wheel. I open force quit; I force quit application, it appears to quit, but then everything becomes unresponsive.

Eventually I try to force quit all other applications, they appear to close, but don't really, then Finder becomes unresponsive, I try to Relaunch Finder, until I only can stare at empty desktop. The only thing that works is pressing button at back of computer to force it to shut down and restart. From past personal experience both at work and at home (am a designer, so always use Macs)...a computer that keeps crashing this way, eventually becomes damaged and has to be brought to store for repairs. This computer is only 2yrs old.

Info:
iMac, Mac OS X (10.6.8)