This could be a PSU or GPU issue, or just a card that doesn't overclock well.
Being able to overclock a chipset is not a guaranteed thing, sometimes you get a card that will oc well without a problem, some might not oc at all without crashing.
How exactly are you overclocking the card automatically, manually? Are you using AMD CCC? Are you changing the voltages? Clock speeds? What are your OC settings vs stock settings?
Generally when you up the core clock speed you also need to bump the core voltage up slightly as well, if you are only changing the clock speed and getting a crash, up the core voltage (very slightly) using the smallest incriment available until you have a stable voltage/clockspeed combination, you should watch temps while doing this as voltage increase = temperature increase.
A few notes: using someone elses overclocking settings may or may not work for your card. Each card will vary and this is why it is important to slowly up the clock and slowly up the voltages. Using voltages that are too high may damage the card.
You can use other people's over clocks as reference points but don't expect yours to function at the same voltage/clock speeds as everyone else has.
This is generally how I overclock: I will leave the voltage at the stock setting and up the clock speed slightly, if it runs stable, I will up the clock speed again, and continue until I get a crash or stability isn't there. Then I will up the voltage and test stability again while monitoring temperatures. I continue this process until I have reached a reasonable speed with reasonable temperatures at full load.
Furmark is a bench I run to max the card out and test stability along with max temperatures. You need to watch closely, overheating a card even once may damage it.
Edited by zingo156, 23 January 2015 - 12:00 PM.