Can you update your signature with your system specs. Include PSU and stuff. makes it easier for people to answer your question m8. As it may not be an issue with the card. it could be some other underlining problem.
But for now, Delete the driver totaly. And use something like CC cleaner to remove it from the registry.
Then download a fresh copy of 12.4 and install that. The driver works fine with a 7870 as im using it with one right now.
If it still does it with a totaly fresh driver install. then we need to look at the cards itself. Is it faulty, is it getting enough power.
Could also try lowering the clock a bit.
I have overclocked mine to run at 1150 gpu clock speed, and 1350 Memory clock speed. And thats with a a 10% power increase.
Try running the card at 1010Mhz on the gpu clock and 1210Mhz on the memory clock. thats the stock settings for a non overclocked 7870.