electrical failure/or fuel?

Astra runs as smooth as! That is until it just stops! engine management light comes on and the engine just shuts down. Wait a few seconds the the engine will start again. Sometimes this will then be ok for 50 or so miles, othertimes it happens every half mile or so. Been in the garage a week where the mechanic has been using it to go to and from work, but he could not get it to do it. On picking it up drove out the garage where upon it promptly cut out. I had this problem on an old belmont. Where we changed every single electrical component. Coil, Amplifier Alternator distributer you name it. In the end I cut a complete wiring loom out of another belmont and wired it into my one. worked fine for about 50 miles where upon it would just cut out completely. Just as if it has total power failure. which is exactly the same symtems that i am having with this astra! Never did cure the problem with the Belmont so sold it on. Do not want to have to do this with this Astra. so if there is any one out there with any ideas? please help! By the way every part of the fuel system has been checked out and ok.ed