Computer worked intermittently, could it be logic board?

I'm going to tell a story about my computer. Possibly some details are red herrings; I don't know. But any advice that can help me differentially diagnose the problem would be helpful.

Preface: Before the computer had this particular problem, I was unable to verify disk or repair disk, and my computer was really slow. I couldn't find the DVDs that came with the machine, so I hadn't made progress on this issue, and then . . .

About 7 or 8 months ago, I spilled some water on my computer. I turned it off, let it dry overnight. It seemed fine. But I carried it to work a couple days later and it wouldn't turn on. I could hear it try when I pushed the button (it's the sound I associate with either the hard drive starting to spin up or the optical drive starting to spin. I don't know what it is, but it's that brief "zz-slip" sound).

Knowledgeable person at work looked at it, said it was either the SMC or the logic board.

I didn't have any money at the time, so I let it sit. I found the DVDs. About a month ago, I plugged it in, charged the battery, and turned it on. It powered on part way (don't remember details), and I kept messing with it, and was able to put the start up disk in. I couldn't do much, so I wiped the hard drive (I had a time machine backup). I decided to reinstall the operating system. I did, and then ran Apple Hardware Test. It indicated no problems.

So I ordered a SDD and RAM upgrade, and installed them. The machine worked beautifully for a couple weeks. I did all of the updates from OS X 10.6 to 10.6.8.

Then, it turned off with no warning, several times. The first times were when I was using skype (skype wouldn't work until I got it the updates). The last time, I was using Songbird (a music player) and iPhoto at the same time. (Firefox was probably running too.)

But as long as I wasn't doing those things, no problem. The machine seemed fast and reliable. However, I went to wake it up today to try apple hardware test again to troubleshoot that problem, and it won't turn on. I've tried resetting the SMC.

Given this grab bag of weird behavior, where should I start to troubleshoot? Can logic boards have intermittent problems like that?