ZionRx :
PSU: Raidmax RX-450K (don't know about PSUs)
To me this REALLY stands out in your system. While the 5770 has a power requirement of 450W it also has a 6-pin power connector requirement. That PSU doesn't have this, but obviously you can use the 4-pin molex to 6 pin adapter. That PSU is rated at 21A on the 12V rail, but honestly I can't find the actual power rating of a 5770 anywhere other than it being 450W. I know my 5870 had this printed on the box, and maybe yours does as well. My BEST guess would be it is your power supply not delivering enough power. I wouldn't put much faith in the unit you have for gaming operations.
In all honestly you don't need much to run a 5770, but something that can deliver the power that a graphics card requires is still a must. If you don't want to break the bank on a PSU, this guy will get you what you need and has a $25 mail in rebate if you don't mind waiting for a check in the mail. It also has free shipping. There is a 500w model that is still more than what you need but it is $6 shipping, but $5 less. 550W is MORE than what you need but that never hurts and it's a good price. If you don't like the red led, just go for the 500W model.
OCZ 550w:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817341022
If you really wanted to you could probably get away with this 400W from Corsair. It has a 6pin and is 30A on the 12V rails. These little guys are REALLY nice and is $50 with a $15 MIR. And free shipping.
Corsair 400W
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139008&Tpk=corsair%20400w
Like I said though, I can't guarantee this is your issue, but it REALLY stands out. Power supplies can really be a funny thing. That is why PC gamers usually stick with the brands they KNOW are reputable and deliver every time. It's one of those things you don't really want to cheap out on, but yet it doesn't mean you can't get a good deal.