2003 Chevrolet 3500 van 6.0 engine auto rwd about 120,000 miles

Hi Folks,A friend of mine uses this vehicle as a taxi—we live in the British Virgin Islands and the vehicle has run very well up to a couple of days ago. Egbert was driving to the village when the clock suddenly changed to 12 midday and the radio started making buzzing and static noises. At precisely the same time the engine started to misfire and run rough and died. It restarted and continued to run rough. He finally pulled the fuse for the radio which made a big difference except that the van now seems to be down on power trying to back up a steep slope with 8 people on board, although it seems to be OK in drive. We have tried to reset the ECM by disconnecting the battery and waiting 10 mins etc, but that did not help at all. He also took the vehicle to a local garage who put it on a machine which showed no error code although the engine check light was on and the car was clearly not running well. The nearest other dealer is in St Thomas and the vehicle is desperately needed for business. Anyone have any ideas, please? Thanks—Gordon