Dont know of any "factory " installed kill switch that is under the dash that is reset-able..this definately sounds like an aftermarket setup .
Have the mechanic check into this and disconnect it,just to eliminate it as a source of the concern.
Intermittent concerns are a nightmare and probably a contributing factor to my hair loss over the years.
You'd like to think that if something was bad,dammit ,its bad...but not always the case with automobiles.
I have had vehicles that would act up for the customer ,as soon as i got it,it would never act up,even if i kept it for weeks.As soon as the customer would get it back,poof,stalls out...
I have had vehicles that intermittently would stall,no rhyme or reason to it,and start right back up..never set a code,never ever give you anything to go on..Even a intermittent fault in an ignition switch can cause a condition like this...
So occasionally you have to try something,change a part.
Have the mechanic test the fuel pressure,test the injectors,"tap " test the ECM and go from there.
Keep me posted...I'll try to help as much as i can...