Could be that the engine management system, which measures water temperature
is complaining that once the car has become warm and the engine is still
running it is no expecting to see too large a drop in water temperature.
This would suggest to me that either you have a large airlock in the cooling
system or the thermostat is stuck open.  I would think it could be the
latter  Coasting down hill the engine is producing little heat but with the
stat wide open the water would cool quite quickly.

You can test for a stuck open thermostat quite easily.

Start the car from cold and leave the engine idling.  Grab hold of the
bottom radiator hose and monitor its temperature.  Is should stat cold/cool
for a long time.  You may find the top radiator hose warms slowly.  Once the
water in the engine block gets up to temperature the stat will open and hot
water will flood into the radiator.  When this happens the bottom radiator
hose will get hot QUICKLY.

If the stat is stuck open the bottom radiator hose will warm up slowly as
the water being heated by the engine is being circulated all the time.

Sadly this process is like watching paint dry becuase nothing happens for
ages, then woosh.  If you take a break for a minute it will then that the
stat opens and you will miss that sudden rise in temperature of the bottom
hose.

Nick /////