Couple of comments -
1. Sounds like good advice on troubleshooting through the clutched fan and into the radiator itself
2. I hope some day to see some independent data on Waterwetter - so far, all I can tell is that is a self licking snake oil ice cream cone - reduce the ratio of coolant to water and add water wetter and it will run cooler. It will also run cooler if you reduce the ratio of coolant to water and don't add water wetter, right?
3. I would think your V8 should take that hill with limited stress - after all, it's not Texas in the summer .... it should normally run a bit about the 80C mark
4. Could there be a blown head gasket that is setting up heat bubbles in the coolant that sweep through and cause the spiking?

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg