What is the connector like at the bottom of mpu board. Maybe a slightly dodgy one?
Connector looks fine . Also, if it was the connector surely the whole column (or row) would be playing up rather than just one switch .
This fault is doing my head in for sure. I have also checked there is no short to earth from the switch , no short between rows and columns either .
I think Rob might be on the right track.
Individual switch lines can be affected by a bad connector.
The Insulation displacement connectors are prone to problems.
You could try carefully pressing the individual wires into the connector with a small screwdriver to see if any change to switch error.
http://www.iobium.com/wpc_idc_connectors.htm
