Please help me!!!!

After I zeroed my router to the work piece and start running the gcode the z axis starts moving to the z home switch.
When the z axis comes down it stops exactly 25mm above the workpiece and the dro shows zero.
It happens 9 out of 10 times.

Sometimes if it starts from the 0.0.0 position the z axis dro skips exactly 25 mm ant then starts counting normally.
When it returns to the workpeace it plunges 25mm down at full speed, it costs me plenty of broken endmills.

I cannot find the problem: I tested the connections of the wiring and all the hardware. and find there is no problem. Why this 25mm???