I'd check the sending unit if everything else looks good. Although the coolant sending unit is a little bit of a pain to get to, its cheap.

My other thought would be the thermostatic fan clutch is busted and the fan is spinning at full speed all the time. On a cold engine, with the engine off, you should be able to turn the fan with a very light drag. If it seems to be locked up, replace the fan clutch. It isn't uncommon for the fluid in them, which doubles as the lubricant for the bearing in it, to leak out and cause the bearings to seize up. In cooler weather the engine may take a VERY long time to heat up if this has happened.