My 2014 RT is Over heating - any ideas on how to correct?

[John Moody]

I have a 2014 RT Limited with 40k plus miles. Highway driving temperature gauge reads normal (just below halfway mark), but when I drive below 35 mph my reverse fan kicked in (minute or 2 later) and temperature gauge goes very high when driving fast again (highway speed) temperature goes back to normal. Dealership hook to the computer and said the water pump is working ok and when they raising rpm above 1400 temperature goes back to normal. Radiator fluid level is ok. Not sure what to do to correct the problem. Dealership is going to flush the radiator and add fresh fluid, but not sure if this will correct the problem. Need help please I missing my ride the last few days.