Makerbot Replicator 2X - restarts after heating
"Im guessing its the mightyboard or the power pack."

Right track there.
Usually this symptom suggests your heaters are overtaxing your power supplies.
Since you swapped extruders already, the heating elements and sensors in those are not likely to be an issue.  That leaves the bed heating element and power supply.

A resistance measurement can help rule out the bed heater but since it works without the extruder, its probably not a severe issue there.

If I had to guess, I'd say power supply.  If you can, rig up a meter to measure its output and cause the issue.  If the voltage drops drastically in tune with your screen going dark, try to replace with a beefier (higher amperage) one.