Mac crashes randomly

My iMac (late 2009) has been crashing randomly every couple of weeks (sometimes more oftern) for possibly up to a year now. It's a weird crash... it just stops, as in I'll be doing something random (eg: from searching the web to Photoshop) and the whole computer will stop responding. I can't change desktop, I can't click anything, the beachball doesn't even show up. It just stops. The music keeps playing until the end of the song, then that stops too. I have to hold the power button and then start up again to recover it - loosing all my unsaved stuff.

It had a hard drive failure last week. I figured this problem was probably related, so when the HD was replaced (on Tuesday) I figured the problem would be fixed. I started afresh, no backup restored.

However, today I was using Safari and my computer froze the same way as before. So knowing it's not the HD, why is my computer failing? I get no error messages, and sometimes the computer freezes when I'm doing really non-memory-hogging things.

I have 2x 2GB extra RAM (from the Apple online store, just over 1 year ago) which I asked the guys at the Apple store to check out when I had the HD replaced, but they said they were good. The Apple store also ran a 'long diagnostic check' on the Mac, and said it was all good after the HD replacement, just 4 days ago. I have hardly any apps installed, and only CandyBar installed from outside the App Store.

Does anyone know what's up? It's driving me mad!