Your mechanic is right there is problem in the fuse box whenever this happen you also check your radiator fan it will also running at full speed. This indicates that there is a loose wiring or carbon deposits on wiring near fuse box.
It also happened to me in my corolla xli 2010. Whenever it happens you can press or shake little bit the fuse box all signs and radiator fan will be normal at off position, and your car will starts again.