Why does my truck die while running like ignition is turned off?

I took my truck to a mechanic to have the timing belt changed. It should have been an easy, routine repair. Upon picking up my truck it has developed a major issue. It will spontaneously die while running. It dies while idling (such as sitting at a red light) or other low RPM situations. My mechanic is baffled and has tried several fixes. Each time he drives it and it’s fine but by the time I get it home it’s dying again. It loses all power as though I cut off the ignition.