1999 LHS overcooling

I have a 1999 LHS that is overcooling, you start the engine it reaches normal operation temp, but as soon as the vehicle reaching cruising speed (30,35, 40, etc) the temp cools to low, so low there is not enough heat to defrost the windows. I checked the thermostat it seems to be working fine. Any suggestions?
Thanks in advance

Replace the thermostat. It is an inexpensive part and will definately eliminate that being the cause.