Next I would try getting rid of Chrome, a known problematic browser, and substituting Firefox or IE. I don't use Chrome because of its reputation for unexpected events.
@DPOC:
I think it's time to make an appointment with your local Apple Store genius bar and have them run their diagnostics on the machine. If it's a Logic Board problem, they'll very likely swap out the machine for a new one. There's no way that all those memory modules could be bad. I've had two iMacs now, the latest being the same as yours, and adding memory to them was a piece of cake albeit a little tough on my thumbs with the 21.5" model.