Printing Problem with 3d Stuffmaker mega iprusa i3

Hi

I have the branded Stuffmaker mega prusa i3.

I have two major problems getting it to work at the moment. The first I will consider at the moment.

I am using repetier for the software control of the printer. I am also using pla on a none heated print bed.

I have calibrated the printer so that when I send it to home, the x,y,z move it back to the far right corner with the print nozzle a fraction of a mm off the print bed. The calibration for the z axis is done relative to an optical limit switch.

When I start a print job after locating its position and slicing it, the print extruder will move to the set position correctly but the z axis moves lower than the print bed, as if it is set somewhere to drop down further than the calibration. The result is I have to kill the print before the print bed is gouged.

I don't know if there is something going wrong with the software, that it adds a -1 or -2 mms to the print in the z axis.

If you are familiar with repetier or the printer and would have an idea on what I can do that would be great.

Regards

Ian