New build POSTs only sometimes, and when it does, W7 installation hangs

[squelchy451]

Specs

Asrock 890GX Extreme 4
Phenom 555 BE
GSkill Ripjaw DDR3-1333 CL7 2x2GB
250GB Caviar Blue
NZXT gamma
550W Antec Truepower

Sometimes, the thing posts and boots from the CD drive, where I have a W7 installation disc. But, whenever i try to install it (tried 4 times now), it hangs at around 40-50% at expanding windows files.

I tried the sticky over at tomshardware about what to do when the computer doesn't post. I breadboarded it and then put it back together, and it worked for about 3-4 reboots, but now it won't POST. Everything turns on, though.

 

[Motorheader]

Are you using the oboard video? Are you screen flicker, beeps, or things like fans running really high speed then settling in when the no post happens? Have you tried just one of the memory sticks in all the slots?

 

[squelchy451]

When the computer decides to POST, it seems to be completely arbitrary. I've tried with one RAM and with 2 RAM, and it doesn't seem to make any differenc.