Running cold on motorway, not thermostat :/

[porka]

I guess it's possible, but it's strange that it happens at 2200 rpm, below that it stays warm, even at 2100 rpm, then if I take it to 2300 rpm it starts to cool. Back down to 2100 and it starts warming again.

Had a look at the flaps but they are not electronic so can't be that.

I had a play with the old thermostat in a pan of boiling water and it opens, then closes as the water cools so basically it was working fine. There is however also a top part to the thermostat which doesn't move with the bottom part but is spring loaded. Just curious what that part is for.