My SDL does did this as well this past spring; the low coolant light would only come when driving a freeway speeds, and then go back off when coming to a stop.

I initially thought the sensor was the problem, but checking the level immediately stopping, did show the level significantly down. However, the level would slowly rise back up.

I added an additional pint of coolant and the light has not come since.

I imagine that the level still fluctuates, but I don't know if this is normal. I keep my eye on it all the time for any other symptoms.

as ever
Paul