space typing apostrophe

Recently the keyboard on my macbook air has been acting out. Every time i press the spacebar, it automatically types an apostrophe, so that when i type a simple sentence, it looks like this: Yesterday Í ´went ´to ´the ´store ´to ´buy ´som ´green ápples

Needless to say, this is highly annoying. I had this problem a few months back, but it suddenly disappeared all by itself (no kidding), until it recently became a problem again.

I have tried:

resetting PRAM ánd SMC

disabling all the slow keys and mouse keys etc.

reinstalling Yosemite

disabling accent charachters

and so on and so on...

The keys all work fine when using an external keyboard.

All help will be highly appreciated.

Thank you in advance,

Mads