OK weird problem here.

Whenever a homing move hits an end stop switch for more than about half a second, the homing move just stops.

End stop switches have been tested and are working fine.

The X and Y axis work because they hit the switches very fast - however the Z presses/releases the switch very slowly.
So Z axis seeks down to the end stop and instead of rising back up and slowly seeking back down - all it does is rise for about half a second then stops there. If I manually press the Z end stop very quickly then it will rise up as far as expected then slowly seek back down.

If I manually press and hold one of the X or Y end stops during a homing move then the axis will reverse for about half a second then just stop there.

Anyone know what could be causing this?
It was working perfectly fine last night and nothing has changed since then.

M119 shows correct state of end stops when triggered and not triggered.

Using Marlin/RAMPS and pronterface to control it.

Thanks in advance;

Hez