I'm sorry to say it, but if you've replaced the AC adapter, and replaced the I/O board with a known-good replacement part, and experienced the exact same symptoms, then all that's really left that could be faulty is the logic board itself.
However, you didn't mention the RAM...I'd remove it and put a stick of known-good RAM in one slot, test it, and if that doesn't work, move the RAM to the other slot. It doesn't especially sound like a RAM issue, but it couldn't hurt to try.
If none of this helps, you might want to take it to an Apple store. I'm assuming it's out of warranty, but they have some pretty amazing diagnostic tools there that aren't available to the public, and a friendly Genius may be able to run some of them on your machine.
If it comes down to getting it repaired, you can send it to this guy:
He does flat-rate logic board repairs on Pros for $245, and has an amazing success rate. I just shipped a 2.66GHZ unibody Pro to him the other day.