I vaguely recall reading someone else had this problem too, but the opto sensor on the X axis is slightly out of alignment with the piece of plastic which is meant to trip it. That is, the piece of plastic hits the top side of the sensor instead of going through the middle. Because the holes for the opto sensor mounting screws are pre-drilled, there was no way to reposition it.
Just thought I would share how I solved this problem.
Take a small flathead screwdriver, and a jet lighter. You can probably see where this is going - heat the end of the screwdriver up so it is hot enough to melt the ABS, then gently rub it on the straight flat area of plastic coming out of the extruder assembly, just before the curve which leads to the flat end which will trip the opto sensor. Next, repeat at the bottom. Do this a few times until the plastic is getting 'sticky', and while it is still hot and pliable, bend it slightly so the flat end which will trip the sensor is still flat, but on a slightly lower plane.
Sorry for everyone who thought this was bleeding obvious, hope this helps someone eventually.