Getting random incorrect error messages on instrument panel of 2001 DeVille DTS

What is your question?

1. After just starting the car will sometimes get an "Engine overheating, idle engine" message when there is no possible way it could be overheating. 2. I have gotten the "Speed limited to......" message but with random speeds including: 1, 26, 80, 90, 127, and a few others I can't remember. I have pictures of some of these because it truly is unbelievable. I think I have pretty much gotten every error message possible at one time or another all unsubstantiated by any hard facts or tests, for instance it will say low range or low fuel but have a full tank, or check coolant level but be fine when I check it, or check gas cap, etc. Now I have a high idle at startup around 2,000 to 2,500 rpms and this continues sometimes nonstop throughout the drive which sometimes causes some pretty hard shifting. Any help would be greatly appreciated.