This is the section of the pushbuttons. Each of them, when pressed, is shorting to ground one connection in a resistor chain. This basically is a variable resistor to the microcontroller on the other end of the ribbon cable. The microcontroller measures the resistance and decides which value means which keypress.

The problem comes when there is another resistance appearing due to dirt or other reasons. Then the whole circuit can behave as if random keys are pressed.

Remove ribbon cable

Measure resistance to ground at this point. Should be OL (open)

If not, measure resistance across pushbuttons. Search for lowest resistance. This is the place you want to replace the pushbutton

The resistances (in the circles) from left to right are 1k, 1k2, 1k8, 1k8, 2k7, 4k7