You didn't mention how long you've owned this car, but I suspect you'll find the cabin (pollen) filter willl be pretty dirty. When you take the cowling panel loose and remove the filter, you'll see that the regulator uses the air flow through the filter to help cool it. If the filter gets really dirty, the regulator fails from heat overload. Common problem on a W140.
When you install the new regulator (a very easy job), be sure to apply the heat conductive paste (will come with the regulator) to the bottom of the regulator when you screw it down. You'll probably find the new regulator has a bigger base plate than the original... again, to make the heat sink bigger.
While you're in there, you can check the brushes, etc on the blower motor.

Don't put the old filter back in... install a new one.

Hope this helps.

__________________
KenP

1996 S500 W140 Coupe