Weird random stuff happening with Ramps 1.4

Hi All,
I've just finished building a Rostock and am in the process of calibrating it but the machine seems to be doing random stuff!

I used the Rich Cat version of Marlin and once the FW was flashed the machine was very slow on the Z axis, It moved the right amount, just pretty slowly. When homing it moves a lot quicker and homing seemed to happen no problem. The next time I used the Z axis it was considerably quicker (what I'd have imagined it to be) but the Z0 was about 0.5mm lower than it was the previous time, I asked it to home again and one of the motors crashed through the magnetic endstop. previously I'd observed what seemed like random changes in speeds and after homing, movement on the x or y axis would put the magnets through the endstops again.
I should also say that I set the drivers (DRV8825) to 0.5v as the motors I'm using are 1.7a/phase, I used 1/32 microstepping and the motors seem smooth, quiet and with good holding torque.

Does this sound like a set up issue or a knackered electronics setup?

Cheers
D


Edited 1 time(s). Last edit at 02/09/2015 01:46PM by maddog7.

I've had some progress on this but have a question:
If I home all axis, this seems to work perfectly each time I do it.
I tried sending a dummy print to the machine, the machine goes to home and when the rear motor hits the stop the ramps re-boots (or that's what it looks like) causing the motor to crash through the stop.
I tried it twice and it happened at the same point twice.
Any ideas what might cause this?
Weird that it does not happen if I manually home all axis but it does when printing?

Cheers
D


Edited 1 time(s). Last edit at 02/11/2015 04:06AM by maddog7.