Opel Zafira 1.7 cdti Diesel flashing coolant warning light

I have an opel zafira 1.7 cdti Diesel, which has been flashing engine coolant warning light in yellow/orange, whenever i start it, without even driving the car. However as soon as I start it tends to go on full speed, even though it is not all the time, I would say that it is most of the time.

Another point is when I drive it up 120 km/h for example, engine coolant warning light stops flashing. But when it stops flashing accelaration also stops, the car does not increase the speed anymore, no matter how hard I press the gaz/accelarator.

what is more worrying for me is the dark smoke which sometimes comes out of the exhaust, and mostly when I stop at the traffic light, for example. so when I stop without puting the engine off. Now when I start the car and drive for 3 minutes and stop at the traffic light, I see no smoke. but If I drive for 5 minutes and stop at the traffic light I might see the smoke.

The fluid levels are fine in the bottle and there is no sign of any leaks.
The coolant temperature sensor connector is also well plugged in.
Does anyone has an idea of what else it could be?