That's a shame, if it was a PC you can run the keyboard under the tap - seriously

One of the common problems with keys is crumbs/grit under them - try giving it a serious battering with multiple key presses with some force to try and crush anything underneath. Failing that, the keys often just ping off if you stick a small screwdriver or similar under them - you might be able to see the problem.
In the mean time, as an alternative to on screen keyboard, try using a spell checker add-on for your browser. Spell it wrong, right click with a mouse and choose the correct spelling.