I get the same problem with remote controls, as well as my Commodore home computers. Each key has a little rubber conductive disk on the underside. When I push a key it touches two copper traces, bridging between them, conducting current.
It misbehaves when tarnish or contaminants get in the way. Many times I've had to fix problems in this way:
1) open the enclosure,
2) further disassemble by unscrewing or unsoldering items,
3) lift the membrane off the board,
4) clean with alcohol or soapy solution,
5) allow parts to dry,
6) remove tarnish on copper surfaces by rubbing with pencil eraser,
8) examine with magnifier, brush away particles,
9) test membrane switches for proper operation,
10) reassemble.