I am yet another one of those who is poor at interpretation of the ES data.......
.... but that doesn't mean I won't say anything
I think the time/engine speed/coolant is about right.
As Mike says, it is idling, and warming up, but not getting airflow.... then it gets moving and the air through the radiator cools it down...... until it gets to the point where the engine is heating the water at a faster rate than the airflow through the radiator is cooling it, so there will be a gradual climb at the sensor....... and then it's all over, and the 'cool-down' lap does what it is named for, and the airflow is now more efficient than the heat input, so the temp drops.... until the speed is down enough to go back to "insufficient" airflow - possibly back at the paddock - and the heat rises, even after the engine is off, due to heat soak from the metal bits.
It looks very dramatic on the graph, but (apart from the bit before the thermostat opens properly), there is only a 12C variation overall.
Nick, did the new reservoir cap stay sealed? and did it release any excess volume at any point?