I have the same problem recently regarding my guage temperature going up to 90 degree. I have a 96 C36 currently at 75k..even in the winter, it only took about 2 minutes of driving and my temperature reach 90. Eventhough it stay there Iam still worry. So I did, like everyone here, took into Mercedes Deaership and other MB shop to have them check it out. They couldn't find anything wrong and say it's normal for Mercedes to run at 90 degree. I told them this is not normal (I know my car), when my car use to run around 80-85,even in the summer.
Been planning to get on the forum to research but never had the chance. Kam am glad am not the only one freaking out about this. Thanks for all the researched and answer. I will take my car in and have them check out all the parts and area you pointed out earlier.
Please let me know if you found out anything else to correct this. I hope my car won't have any more problems...this is my first Benz, so I hopeing to enjoy this ride for a long time.