ATI Radeon HD 3870 problem!!!

Ok, I've had a problem with what I think is my graphics card for a while now and it's been getting worse and worse. Here are my system specs:

mobo: ASRock ALiveNF7G-HD720p R5.0 AM2+/AM2 NVIDIA GeForce 7050
Processor: AMD Athlon 64X2 6400+ 3.2 Ghz
Memory: 3 GB DDR2 667 RAM
GPU: SAPPHIRE 100225L Radeon HD 3870 512MB 256-bit GDDR4 PCI Express 2.0 x16
OS: Windows XP SP3
Monitor: 20" Widescreen LCD 1600x1050

So it all started after I put in a new graphics card and fans. My old 500 watt power supply blew so I got a new 550 watt which seems to be powering everything ok. I'm not sure if my motherboard or graphics card is the was damaged though. The problems I have are:
-blank screen on boot every now and then, which I then have to manually power off the comp and turn back on
-screen is off center to the right frequently when I get into Windows, which I fix by changing the resolution and then change it back
- While playing games (Kotor I & II, Fable) if I turn the some of the video options even on medium I will get colored dots or checkered boxes across the screen and just recently crashes.

I've tried setting the primary graphics adapter to onboard in BIOS, but once I get to the Windows loading screen I get blank. Help please???