Printer behaving erratically during print from SD

I'm using a CTC printer that I've just built. It mostly runs fine, but every so often will pause in the middle of a print, and home the X or Y-axis (occasionally both together), and then return to the object and start printing. This is mildly annoying and sometimes leaves a bit of string hanging off the edge of the object, but I can cope with that.

More rarely, halfway through a print (usually a longer print) it will just retract all of the filament out of the extruder,
and then carry on with the print - but no longer producing any plastic, so I get a half-finished print.

Both of these effects can occur at random points through printing the same object, so it doesn't seem to be directly related to the gcode. It doesn't seem to be related to retract events either - the last time the filament was ejected was halfway through a large mass of infill.

Does anyone have any ideas what is going on here? I am wondering if it is a glitch in the firmware, but could it be the power supply? I have a 180W LED power supply. (The quality of components is not great - it was a fairly cheap purchase of eBay; I've already had to adjust for the fact that they have used 50k thermistors rather than the standard 100k ones).

I'd be really grateful for any light that could be shed here,

Phil