my car engine dies at any speed,after a moment it starts up and run normal.this on 1991 Mercedes-Benz 560SEL

the problem started off by happening every once in awhile now its much more frequent

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. Start with the basics, check for ignition spark on cranking when it fails, check fuel pressure and volume when the car fails hay have a fuel pump going out or could even be electrical, we would need to experience the problem and catch the car in a "failed" state.
Your engine uses mechanical Bosch fuel injection similar to that used on Volvo, Peugeot, and VW. It was a great system but was easily damaged by water in the fuel or dirty fuel. Few younger guys know this fuel system well any more. Find a trustworthy independent Mercedes repair shop they will be familiar with this car. Check the basics first. These were a great car,many still on the road with huge mileage.