'97 w210 throwing off aux cooling belt
from time to time my 97 w210 aux cooling fans throw off the belt driving the second fan. I've replaced the belt thinking that was the problem. No joy! The problem returns sporadically. I don't see any damage to the pulleys.
Any suggestions?

Make sure that the pulleys are in line. If not, it could be that the frame is bent. The belt should stay on the pulleys. I've seen one report of a thrown belt getting caught in the drive pulley and putting a hole in the condenser. Not that these condensers don't seem to develop holes on their own, but they need no help.

The mounting frame is plastic, maybe it flexes? Is this OEM? is it supposed to be metal?