1998 Chevy Cavalier Cooling light

Last summer my car started overheating. I was loosing water and I would not have water pressure. I replaced the thermostat and the water pump. I am now not loosing water and I have adiquate pressure. When I drive the car around town everything seems fine. When I get onto the highway or over 35 mph, my temperature begins to drop. I am getting hot air from the heater but once the temp drops below normal, the air become cold. I then proceeded to replace the thermostat again thinking I had a bad one. This did not make a difference. My low coolant light remains on even through I have enough water. I checked the water level sensor and its plugged in. Could the light and the loss of temp have something in common? I have bled the water lines for air pockets but I still loose heat at high speeds. What do I try next?