The machine will need dismantling - are you upto it?
The key board is a vynil membrane with contact points that touch a circuit board to make contacts.
Sounds as though some liquid has got through and dried out between the membrane and board below.
If you quake at the thought, get your self a copy of
MicroMart from the Newsagent - comes out weekly on Thursdays.
Lots of repairers, parts suppliers etc there.
Will cost you though.