Sounds like the radiator fan to me. There is a recall out for that part (well, I think it's more like an extended warranty, something like 10 years or 100,000 miles). I had the warranty done on a 95 Mystique last week, for the exact same problem. Ford replaced fan, wiring harness, etc. I'm having the same repair done today on a 95 contour. Same exact problem you are having. In traffic, on hot days, the car ran hot. The fan was not coming on to cool the radiator. I suppose when I was driving at highway speeds, there was enough air flow to cool the system. Anyway, Ford did the repairs at no cost.
