May be an issue with the keyboard connection to the logic board, if "6yhn" appears. The other symptoms could be several things, including the logic board. You'll have to take it to an Apple service center for diagnostic work. They can see what's wrong and tell you what the cost would be for repairs (it's out of warranty, right?).
If you don't know one near you (Apple Store), you can call Apple or look at their website for a service center near you.