MBP 15" Early 2011 Keyboard issues - some keys register as not pressed or as several random key presses at once

I just bought a used MacBook Pro 15" Early 2011 (2.3 GHz Quad-Core i7, 8 GB RAM, 256 GB Apple SSD, High-Res Antiglare screen), without any warranty or AppleCare left.

The seller had just wiped the harddrive and reinstalled OS X 10.8.2 Mountain Lion before selling it.

I tested it with Apple Hardware Test and in various other ways before purchase and everything seemed fine, including what is now the issue - the keyboard.

After I got home, I used the computer for about 10 minutes, then all of a sudden the keyboard started acting up. Some keys seem okay, whereas others have major issues.

I opened Keyboard Viewer to see what key presses are actually registered. The 'b' key for example sometimes registers nothing and sometimes registers as 4-5 presses (of other keys) at once (x, z, v, Command for instance), and the 'n' key similarly registers either nothing or outputs 4-5 various letters/characters at a time. Caps Lock sometimes turns on/off when other keys on the keyboard are pressed. There are more keys with similar issues as well.

These are some of the things I have tried to resolve the issue:

• Reset SMC and PRAM - keyboard still not working properly.
• Reboot in safe mode - keyboard still not working properly.
• Boot up, holding Command-Option-P-R, into OS X Utilities, start Terminal and try keyboard there - keyboard still not working properly.

I do not have "Mouse Keys" enabled, and my language is set to English.

Right know I'm running out of things to try, short of opening up the laptop to check for loose cables etc. I don't have the right screwdriver, so can't do that at the moment. Next would be handing it in for extortionately expensive repairs. I'm located in Sweden, so just getting the problem diagnosed is around 130 USD, labour for changing keyboard would probably be about the same money again, not including parts.

I have connected a wireless PC keyboard with a USB dongle, and that works fine, so the problem seems to be related to the built-in keyboard.

Any suggestions that might help resolve this problem are more than welcome!

Regards,

Nicklas