I cleaned my sensors as Steve suggested awhile back trying to fix an ABS issue on a 126 car. Helps if you have the 126 CD for a description of where things are, though it's not that tough.

In my case, a single 5mm allen head holds each sensor in. This is to be found on the inside of the hub. You gain access from the front and down underneath. The sensor has an electrical line that routes upwards through the wheel well.

Once the 5mm allen head is out, you can start rotating the sensor to loosen it. Eventually it will pull out. You can clean the gunked up end of it with brake parts cleaner.

Won't hurt to try.