HELP! Z4 Wipers stop without warning!

[phong]

I have a 2003 3.0i z4 with automatic rain sensors.
I turn on the wipers to any speed setting (including the automatic sensing position) and they work fine.
However, leave them running for a bit while i am driving and suddenly they just stop working. Changing the settings of the wiper speed or pressing for a water jet makes absolutely no difference, the wipers will not work until i stop the car, switch the electrics off and on again.
I got caught out whilst on the motorway and it was raining heavily, luckily it was at night and not much traffic about so i could stop and restart the car on the hard shoulder.
What could it be?
The wipers also stop too soon when they reach the bottom of the windscreen (they sit too high) if i have them set on the automatic rain sensing position, but if they sporadically go back to the correct place if you leave them running for a while.
Could it be that my windscreen wash tank is empty ( it has a leak and i havent bothered to fill it up recently).

Thanks

[roofer]

I would look at the wiper relay, E39's used to have a very similiar problem which was the relay. Sorry i can't be of more help.