Why does my temperature gauge read hot, but engine is at a completely normal temperature?

I have a 2005 Honda Accord LX. A few months ago the temperature gauge
started reading hot. I can get in my car and drive on the highway or around
town and the temperature gauge will read completely normal, but after about
20 minutes the needle will start to rise getting as high as the red. The crazy
thing is that when I pull over to check the engine the temperature is completely
normal. Ive taken it to two mechanics...The first mechanic checked the radiator
and fans, but could not diagnose the problem. He also noted that it could just
be a bad sensor. The second mechanic I brought the car to diagnosed the
problem as a bad radiator noting that the fluids were not flowing properly.
When I went to buy the radiator the guy at the radiator shop said that the
problem is not the radiator, but either the fans or the sensor. He said that he
didn't want to sell me a radiator when I didn't need one. Just wondering what
you guys think about this? The fans are fairly new and the car only has
136,000 miles.