I need to know what might be wrong with my husband's 1989 420SEL Mercedes Benz
My husband's 1989 420SEL, keeps shutting of even when you are Driving, what could it be.

Mix luxury appointments (durable leather seats, real wood interiors) with a smooth, nearly unmatched "ride," and you have the Mercedes-Benz 420-Class, specifically the 420SEL sedan. In fact, many drivers believe these early-'90s MBs to be more luxurious and better built that present-day Mercedes, and some consider them to be among the finest sedans ever built. They were, to be succinct, rock-solid automobiles.
The 420SEL was long and heavy, but a 4.2-liter single-overhead-cam V8 that generated 201 hp, coupled with a 4-speed automatic transmission, kept them humming along the highways very nicely. The 420 provided ample room for occupants (nearly 40 inches of legroom in the rear seats, for example) and comfortably seated five.
Standard equipment included anti-lock brakes, power locks and power windows, air conditioning, and airbags for the driver and passenger. Other standard and optional features included automatic climate control, bucket seats, power steering, power telescopic steering wheel, cruise control, headlight cleaners, power heated windows, and alloy wheels.
My husband's 1989 420SEL, keeps shutting of even when you are Driving, what could it be.
Thinking it was a linkage problem after my car wouldnt start in park, I then began starting it in Nuetral. After sometime, knowing I needed to have this checked, the car refused to start n Nuetral. A relay in the fuse box area showed signs of igniting, but until I know for sure what my options are, I do not want to continue checking things and cant afford a specialist. please help
Air Conditioning
Automatic climate control front air conditioning
Airbags
Dual front airbags
Audio System
Antenna Type: power · AM/FM radio
Brakes
ABS: 4-wheel
Comfort Features
Steering wheel trim: leather
Convenience Features
Memorized settings: driver seat · Steering wheel: power telescopic · Sunshade: power rear window
Exterior Features
Paint: special factory paint
Mirrors
Exterior mirrors: power
Roof
Moonroof: power glass
Seats
Front seat type: bucket · Heated: front seats · Passenger Seating: 4 · Upholstery: cloth
Tires and Rims
Wheel type: aluminum alloy
Windows
Front wipers: intermittent