4wire analog touch problem

i have connected a 4wire analog(resistive) touchpanel to my cop8
microcontroller

for measurement the supply is directly done with the cop i/o pins. no
problem in generall.

but on measuring the voltage in y-direction i have a drift of the measured
voltage. y-voltage on the left side is different to
the y voltage on the left touch side if the y-position is constant.

this effect is not visible on the x-measurement.

anybody a solution ?
which switching fet are used usually in this application.

regards chris