Low coolant light

Engine Cooling problem
2000 Buick Century 6 cyl Automatic 75000 miles

when I start the care the low coolant light comes on for a while when the engine warm up it goes off.
The coolant level is good I tried a new radiator cap did not work

The car is a 99 buick century, 3.1 ltr. I just replaced the coolant temp sensor, throtle position sensor and the idle air control valve. I replaced the thermostat with the proper 195 degree replacement and refilled the coolant to proper levels and bled the cooling system. Occasionally we are still getting the LOW COOLANT LEVEL LIGHT. It flashes for 3 times. Sometimes 4 groups of 3 flashes and sometimes 3 groups of three flashes. The light remains out at start up but comes on with the temp guage reading 3/8 ths. This has always been the normal reading on the temp guage though. I have double checked the coolant levels in the overflow tank while hot and in the radiator while cold. I know the thermostat is woorking properly because the upper radiator hose has fluid flowing through it at approximately 196 degrees.( This is the hose that goes from the radiator to the thermostat housing)

I have a condition where the low coolant level light is coming on but the levels are good - hot and cold - and I can detect no leaks.

I checked the oil and there is no coolant in the crankcase, and seals have been replaced previously (which is a long term problem with this engine due to seal degredation from DEXCOOL antifreeze.)

The condition is only apparent after approximately 10 minutes of 55mph + driving on the interstate. Light will then stay on until the car comes to a stop at which time it turns off.

I suspect a faulty sensor but was curious of any other opinions before I pursue that route.

Thank you.

I also put in some stop radiator leak!

3.1L SFI OHV 6cyl

My low coolant light is flashing and the systom is full