From XFX's specs page on the 5770:
450 Watt or greater power supply with one 75W 6-pin PCI Express® power connector recommended.
If this is the PSU, it only has a single 12V rail with only 18 amps. Thats horrible for a modern machine that uses 12V for almost everything. My bet would be insufficient power causing the graphics card to cut out.http://www.hardwaresecrets.com/article/Thermaltake-Purepower-430W-NP-Power-Supply-Review/332/9
The main problem with this power supply is that it can’t deliver its labeled power. It is, in fact, a 350 W power supply.
at a load of about 380W, the PSU’s switching transistor overheat and burned down in the power supply. It was so hot that the insulating plastic spacer under the screw the transistor was fastened with melted.
OUCH! Get it out of your computer before it damages something.