Intermitent engine shut down

I have a 1990 Mits Pajero 3.0 V6 Auto superwagon, it has been a great car for many years, but of late the engine will just stop while driving or at idle. It has also been difficult to start. It will wind over but will not fire. Sometimes when it does start it will run very rough like it's running on 3 cylinders. It has been in and out of the mechanics for the last 2 months (more in than out). The computer, relay, coil, alternator, numerous connections on wiring harness have all been changed. Still no joy. Has anyone experienced this problem or have any ideas on the cause.