FW hangs/reset on long lines?

So, i am having this issue i can't wrap my head around.
Printing parts with long Straight lines causes the firmware to either reset (watchdog i assume) or just hang, sometimes with heaters on full throttle! what happened to the dog?

here are my findings:

A single wall circle D 250mm, prints fine, as tall as i want, great.
A single wall square 250x250mm always fails, usually within the first 10-20 layers.
SD or USB print makes no difference.
placement on the bed makes no difference.
Print speed makes no difference.
it does not fail when using dryrun (no E moves or heater).

The printer is a homebrew cartesian with a RAMPS+Mega2650+DRV8825(Chinese) running 0.92.8
tripple mixing extruder.
SSR controlled 240VAC heatbed (280x500).
Xbox360 PSU (12V 203W)

ANY ideas would be greatly appreciated.
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