Nothing surprising here. You bought a card that is already over-clocked per factory default and may barely have a safety margin left, both in terms of actual processing as well as thermal behavior. It's almost natural it would cause issues under extreme loads and tweaking settings downward would be the remedy...
I concur with Mylenium. Overclocking may work for some things but when you're trying to read/write to memory or disk and you start pushing max buss speeds you'll end up with errors. It's not a good idea if you rely on the gear to make a living.
That's why the engineers set standard clocking rates for systems. It's a part of the quality assurance quality control system any manufacturer has to implement to stay in business. Build a zillion MPH capable whiz gadget and sell it to the public touting the zillion MPH it's capable of and you'll have some that are pleased even though it will only last an hour at those speeds. Limit the gadget to a hundred MPH and it will last 10 years and start every time. You've got a choice. Something reliable that will last and please the greatest number of customers or something that only a handful who want the thrill of going a zilliion or who have the deep pockets needed to buy another every time the whiz gadget breaks down.