Geeetech

Did you try Arduino 1.0.1?I guess it will fix it:
http://www.geeetech.com/wiki/images/a/a ... indows.zip

Could you post some pictures or a video about the issue?

So i have a question: Did you modify the wrong direction of your bed?'Cuz you said it moved opposite to the endstop when homing? Then concerning the layer shifting,check these tips: You'd better check the tightness of your X axis belt(not too much loose or too tight),Y axis driver A4988(try increasi...

Did you solder the 3-pin connector in the place of Z_max as the manual describes?

The step for the motor is 1/16 step,concerning modifying the firmware you can refer to this:
http://solidutopia.com/marlin-firmware- ... ide-basic/
And first download the firmware for your ProC from here:
viewtopic.php?f=20&t=17046

The wiring is similar to the manual describes,your box contains the motherboard GT2560 RevA.
So i'm afraid you may need to make some soldering.

What an unforgettable experience

Well, i just found there is no Mecreator in the "Printer Type"...
I think you can try the following firmware:

It should be normal as expected,'cuz the printable radius for it is 100mm.

Maybe you can try the firmware from here:
viewtopic.php?f=20&t=17046