My 1986 300sdl is doing the exact same thing. I took off both front sensors and they were very dirty with lots of metal filings on them. I cleaned them and sprayed brake cleaner into the holes where the sensors live. It looks pretty clean to me. This did not help. It still does this on occasion.
I have not pulled the rear sensor and cleaned it yet.
On an interesting side note, my alternator is not putting out the correct voltage. At ilde with a?c on and l;ights on and rear defogger on it barely puts out 11.5 volts. At 2000 rpm it barely gets to 12 volts. I have ordered a new alternator. I wonder if the voltage to the ABS computer is too low at idle speeds and throwing the system off. In hot sunny Louisiana, my a/c is always on and I find this happens mostly after I first drive away in the morning and I come to my first stop. Occasionally my ABS light will flash on at idle. My Overload protection relay is brand new.

I am going to change my alternator this weekend and see if this changes anything. If it still does it I will pull the rear sensor and clean it.

John