While driving accelerator cut out and revs dropped to zero?

At first the accelerator cut out for a split second and carried on. When driving accelerator cut out and revs dropped to 0 which made car roll to a stop then struggled to start for a little while. It did this now and again but became more regular. It’s happening more often and takes longer to get it started. Coil pack has been changed and the crankshaft sensor is working as this has been checked. It was in the garage for about two weeks, and they couldn’t figure it out. There is also a little black box behind the glove box which constantly flashes, this has been added by the previous owner but I have no information to what it is or what it does. Please help