Z axis going nuts on my R2E3

[Creative Tool]

I changed my boards around but did not find a problem.The z axis will repeat accurately a few times them start stepping itself down .Depths are impossible to hold accurately. I am going out on a limb by saying that i suspect the encoder is bad.?It has Powertron Contraves steppers. Any thought and help would be geatly appreciated since this machine really needs to run to get jobs out the door,Chuck.

[replicapro]

one time my y axis went crazy, and it turned out to be metal cuttings stuck in the encoder cable