For record, as this post is quite old:
I've had the same problem on an Amilo pi3525, after four years of trouble-free service: keys Return, e, r etc. suddenly not working. At other times, another group of keys (del, esc etc). Or all working without warning.
My problem lied in the keyboard connector: I believe that due to overheating, the heat expansion made the connection faulty.
Fujitsu-Siemens are very good designs, in the sense that everything can be dismounted with screwdrivers and accessed easily without the need to remove any glue/seals etc. I removed few keys to access keyboard screws, unmounted the keyboard and cleaned the connector.
Also dismounted the fan from the back, cleaned the fan and cooling sink (clogged by a half-cm of dust in the fins), put some thermal paste where needed: now I can barely hear the fan working.