7 Keys weren't working, and now they're working on-and-off?

Device: MacBook, Late-2009 Unibody (Polycarbonate White). Last three digits of serial number are "8PW".

OS: OS X 10.8.2 Mountain Lion

Problem: Last night, whilst playing and talking online on Minecraft, the keys E R T Y U I O suddenly stopped working entirely. I tried quite a few things to fix the issue, but none seemed to work. Later that night, and this morning, the keys are MOSTLY working, but occasionally one of those 7 letters doesn't work the first time I press it.

What I tried:

- I tried restarting the computer, obviously.

- Showing the keyboard viewer to see if the keys registered being pressed; (they didn't.)

- Making sure the battery was fully charged (It was plugged in and at 100%, but has shown the "Service Battery" alert for almost a month now, with no issue other than slightly shorter battery life.

- Booting from the install disk and trying to type those letters into the terminal (only those 7 keys wouldn't show up, still)

- Plugging in an external keyboard (all the keys on that work fine, and I can type and use the keys as normal)

- Removing one of the keys and cleaning it out. (no effect)

- Ensuring slow keys was turned off. (It was all along anyway)

- Tried holding down shift, alt, ctrl, cmd and activating caps lock (and many combinations of all these buttons)

- Running a basic hardware test, by rebooting the computer and holding D.

I'm not very technically advanced with computers, I'm rather about average. (So telling me to do X without explaining how may just yield more confusion.. It's a problem I have wherever I search online for help. I find solutions, but don't understand how to perform the solutions.)

Anyway, I digress.

The keys are mostly working now, except for the occasional "misfire". Would you guys have any idea about what the problem could be?

No, I'm no longer in my warranty, and I live over an hour's drive away from the nearest Apple Store (UK). I'm also not qualified or confident enough to install a brand new top case myself.

How much (at an estimate) would an official and / or third party repair cost?

Thank you for your time, and I really really hope someone might have an idea with what's going on...