Check with MSI and AMD and see what the recommended minimum wattage is for your video card.
I suspect that the psu can't deliver stable power. How old is your power supply?
Even if you don't do any overclocking, I think that you should have quality power supply that is rated for 650 - 750 watts at 45o (C) or higher.
Keep in mind that you should never run a power supply higher than 80% of its rated power output for extended periods of time, especially when gaming. Sites like HardOCP and TweakTown report total system electrical loads when they review video cards.
Your power supply is the foundation of your system, don't buy or use a low quality cheapy power supply.
Many cheapy power supplies can't deliver their full rated power.