Mercedes stalling when driving

I have a 1992 Mercedes 300E. This has happened 3 times now. The 1st time it stalled after driving in town for about 10 mn at low speed (35). I had to push it out of the way. It would start again but as soon as I put it in drive it would stall. My mechanic told me to let it sit for an hour an try again. I did and it worked. Started fine and never any sign of what had happened. Then 2 months later it happened again but this time I had driven on the highway for 15 mn at high speed, taken an exit and whe traffic slowed down at the first light, the car stalled and, same thing, would start again but would stall when put in drive. I had to wait for 20 mn until it then started and drove off as if nothing ever happened. Thinking it was dirt in the gas tank, I filled it to the top. The car worked fine. No sign of any previous problem at all. Then a week later it happened again. The tank was more than 1/2 full and it happened at the exact same location at the first red light after the exit of the highway. The garage told me it didn’t sound like an injector problem. I have put a total of 3 injector cleaner bottles in the gas since the 1st incident in December. I only drive the car on the week-end and it always happened on the 1st day that it is driven after sitting for a few days. I am terrified that it would happen again. I thought my life was going to end yesterday in traffic. I am going to ask the mechanic to change the gas filter but could there be another cause to this problem? Someone mentioned the gas pump. I read on the blog about the fuel injection pump, a coil, an electric problem, vapor lock, or a fuel pump relay. Any help would be greatly appreciated. Oh, one more thing. Yesterday, I think I was hearing some kind of hissing sound after it started again and it kept doing it all the way to work. I don’t know if it is related. I only heard it because the window was down.

Thanks!!!