Car shuts down while driving?

[hdon]

I hope someone here can help me.

Recently, when decelerating or braking (for a light, stop sign, or just to slow down to make a right turn, say), my 1990 300E just shuts down, and all the lights at the base of the dash display (oil; battery; wiper fluid; etc.) come on.

After this happened 3 separate times, I took it to the shop that normally services the car. They couldn't get the problem to demonstrate while they test drove it, but after looking under the hood recommended a full tune-up, guessing the problem might be caused by worn sparkplugs, etc.

Full tune-up was completed 2 days ago, and the car was running beautifully.

Then this morning, it cut off again when I slowed to make a right turn.

In each of the 4 instances, the car has started right back up again.

My regular mechanic is stumped. Anyone have any suggestions of what to look for next?

Thanks in advance.