Echo temp light comes on at 60mph

My 2000 Toyota Echo has a strange problem. At 60 mph (+/- 5 mph) the temperature light starts flashing. If i slow down it stops. Coolant level is fine. Car doesn’t seem to be overheating. Radiator fan is working. With radiator cap removed coolant is not moving when the car is cold, but heats up and circulates as the car gets to temperature .I have heat in the cabin. Please help.

This can be caused from something as stupid as a faulty coolant temperature switch for the light. Have the switch replaced to see if that stops the light from flashing.

Tester