Macbook won't boot unless I hold power 5 seconds

I have a Late 2008 Macbook Unibody that has started having issues cold booting and waking from sleep. It doesn't do either correctly. From either state, it will only boot if I hold the power button for 5 seconds until the sleep light flashes quickly (if it's sleeping, I'll have to hold the power button till it turns off, then hold it again to get it to boot). Restarts work as expected.

Things I've tried: Reset SMC, PRAM, replaced HD, reseated and tried different RAM, using one slot at a time. Nothing helps.

I did notice that the lower RAM was initially not snapped into place, even though it appeared to be working correctly. Not sure if that could have hurt the logic board somehow.

My next thought is to try a Firmware Restoration CD, since the only way it will boot is when it appears to be attempting to update the firmware (flashing sleep light).

Any other ideas?

Cheers