Wonky rain sensing wipers

[Speed addicted]

The rain sensor on my 2001 X5 appears to have intermittently stopped working, the wipers still work on other speeds but park about 2" from the bottom of the screen, they sometimes stop in the middle of the screen if you use the rain sensor.

They go onto a very slow intermittent setting when acting up, about 50 seconds between wipes, then they wipe twice.

So, where do I start looking for the culprit? I've cleaned the sensor with no effect.