Z-axis trouble

I have for a few days having this problem when I slice some obects and they are ready for printing it zeros X and Y as it should be the Z ais goes down and then up and then more up until I stop it.
And then i will try to print in mid air.
Any suggestion is appriciated thx.

Got it fixed by deleting the Repetier and a full shotdown of the computer and the reinstall the Repetier, so now it works.