Quoted from barakandl:
Bad resistor(+cap maybe too) networks in the switch areas of the MPU are fairly typical.
Second that....this is the most likely cause
If your phantom switch problem is on the CPU board, don't forget to look at the 1k ohm resistor pack SR10 on the CPU board. When this resistor pack goes bad, it can cause intermittent phantom switch closures. Use your ohm meter, and test the resistor pack. If in doubt, just replace it.
It looks something like this