Sounds like it could be a mechanical fault Keyboards work as a matrix of switches and if there is a problem with the interconnections it can sometimes cause apparently random failures such as yours. If you were getting the wrong characters I would suspect a software issue, (which of cause it still may be),
My best advice is to see if you can find a local Tech that gives free estimates for repairs and let them diagnose it for you, if you were dealing with a desktop it would be a simple swap out of the keyboard but laptops are not that easy to either get at the keyboard to check/change or to pick up a keyboard for testing.