When i had my coolant flushed ford put in 50/50 antifreeze and water. My guage was moving up but wouldnt ever over heat, would just go back down after a few seconds. Brought it back and they did something like 75%water/25%antifreeze and the problem totally went away. I then added Red Line Water Wetter and that itself kept the temps slightly lower. It could be a sicking thermostat, there cheap and easy to change out. Are your fans kicking on?