Black MB won't boot

I have a black MB 2.16GHz from 2007.

When I switch it on I hear the CD drive partially engage, the fan comes on, but the screen remains black. This happens both with battery, and AC. I have to do a force shutdown by holding the power key. Usually after about 10 times of switching on/off I eventually get the apple boot screen, and then a further 10 times after I get the computer on. It takes approx 20mins to switch my computer on each time, and if the lid closes it will not wake from sleep. In these 20 or so attempts the computer progresses a little further through the boot process.

Because my computer still *sort of* works I didn't think this was a logicboard issue, but now I'm not sure.

I'm rather at my wits end! I've reset SMC, tried swapping out RAM, reset PRAM, tried diff batteries, booted from AC only, full battery, low battery, and still no improvement.

History-wise: I had an inexcusable tarmasalata spill 2 years ago, and replaced the keyboard with no issues. Over the past 2 years (although not recently) I have upgraded RAM and HD. I've lost the startup discs so unable to do a boot check.

I've read about similar incidents of boot problems before, but never one where the computer can be resurrected through repetitive on/off attempts.

Answers most gratefully accepted! Thanks for your time.