Richard,
Your problem most likely is the water level is slightly low. What happens is the water is high enough to show full in the sight glass even when cold, but when the water is cold or cooled down while idling in the rest area and you throttle hard to enter the highway, because the sending unit is on the front of the expansion tank, the water runs away from the sender just slightly which is enough to set the alarm. Add a couple cups of water and you will no longer have that problem.
Ron B 07 Camelot 42PDQ