I'm having the same problem with my 87 300E. The guy I bougt it from must have charged the system up right before he sold it, it was freezing on the freeway and did just OK below 40 MPH. Recently (2 months after purchase) it doesn't even do good on the freeway. I removed the low side cap to try and put more freon (134) and noticed that there was dye around the low side valve, so someone knew there was a problem with it before!

Anyway, is the fan working properly? I believe there are two settings. If not, I think it's the #7 fuse, try replacing it as there could be some corrosion between the fuse and contacts. In my case, I believe the problem is bigger and I'll have to have it diagnosed professionally.