MacBook 5,1 (unibody) getting kernel panics if memory bank 0 is populated
You've covered pretty much all the troubleshooting bases. If it's not a problem with the RAM itself, and it's not a problem with the firmware or software, it's almost certainly a hardware problem.

You could try running the Apple Hardware Test, but it's almost certainly a problem with the memory controller, which as far as I'm aware means a new logic board. Take it to an Apple Store or Authorized Service Provider.