My son is an Apple-addict (and I work for IBM!), and he advises that it sounds like someone spilled some liquid in or around the keyboard. Now the Apple stores can tell because they use a special ink which reacts IMMEDIATELY with any kind of liquid even if you blow dryer it dry. He says that since the motherboard on the apple is an all or nothing invention, you are probably right in assuming its the motherboard. We know this from personal experience when I spent $1700 on a new Mac Book only to have his friend spill some (and I mean a LITTLE) diet sprite on it..and even though we were able to get it up and running from time to time, it was undependable at best. So.. my advise is to get an authorized Apple dealer (not the Apple store) to look at it (ie Geek Squad) since they are cheaper. For about $50 they'll either tell you you need a new motherboard or something else..but (and I am sorry to tell you this) dont hold out a lot of hope for something less.

Instead, go with plan B. IF you are lucky enough to own your own home, you are likely covered via your homeownders insureance by simply telling your insurance agent that a pipe broke under the sink and water got on the Mac.
You'll have to pay a homeowners deductible, but you'll get a NEW machine for a WHOLE lot cheaper than a new machine, plus you get to keep the old one and tinker with it..you can buy used motherboards (or some offer repair services), so now you have TWO, and you can sell the repaired one to recover your expenses. If your a renter with renters insurance it should work the same way.

Again, I am only passing along the wisdom of a 17 year old who can make his Mac Book sing and dance the bassa nova ! I hope this helps and wish you the best .. If you want to talk to him send me a pm, and I'll give you his number..

Regards
Ed in Metuchen, NJ