There seem to be a rash of these problems, I guess it's the weather, here's what my car is doing.

One cold morning my 1987 190E 2.6 would not start, I left the car and came back later and jumpstarted it, it started fine, I took it to a mechanic who tested the alternator and battery to see if they needed replacing, his equipment said they were fine, I was able to start the car yesterday and today, the temp was about 29 degrees, after about 1 hour of fast driving, the car died on me while I was coming off an exit, I put it in neutral and it started fine, at a stoplight, it went dead again, I restarted it. I managed to make it home without the car dying on me, can someone please tell me why it's doing this.

Thanks for any advice you can give.