What could cause failure to boot/serial output

[gregcope]

Hi All,

I have an issue with a GPS shield I am making (well failing).

Expected behaviour -  Serial output as setup is run, and then power on a GPS via a P Mosfet (NDP6020P) - the GPS flashes an LED as it tries to get a fix - see schematic.

Actual behaviour - when it boots the LED flashes a few times as normal, but no serial output or other activity (ie no GPS LED) - as if the boot is stuck.

This is with a MoteinoMega (upside down), OSHPark board, Adafruit ultimate GPS board.

If I unplug the GPS, it boots and carries on as normal - plugging in the GPS usually (not always) results in it carry on, and the GPS working until switched off.

What could I have done in either design or assembly to affect this?

Any ideas on what might cause this?