I had a similar problem on my 94 E320. The fan would work and then at some point it would stop for no apparent reason. With help from this forum, I found out there is a strip fuse on the driver side of the engine compartment, near the master cylinder, in a small black plastic enclosure with a flip top. This fuse is for the fan. After looking at the strip fuse and how it failed (hairline crack) I believe that temperature changes under the hood cause the intermittent nature of my fan operation. Sometimes it would make contact across the crack, sometimes not. I temporarily replaced it with a lug style fuse from Auto-Zone and my fan worked great. I later got the proper strip fuse so I could close the flip top and have had no problems since.

Hope this helps,
Dave