since u switched mobos im thinking its the PSU which is half way dead . even tho it powers up on older system doesn't necessarly mean that it WILL power up a newer system because systems these days require more voltage and Watts.
try a different PSU with 15a or more under the +12v range and see if it boots up.
try a different PSU with 15a or more under the +12v range and see if it boots up.