Clipper hunting at idle

Hey peeps, I've just replaced my girlfrind cylinder head on the clipper which went fine and for first couple of days drove fine but still seemed to run hot ish but didn't overheat. So I suspected the thermostat wasn't opening at the right temp so I got a new one from VW and filled the system with G12 50/50 mix as recommended. Once I started the car after this the idle was hunting i.e. revs bouncing up and down? Now I pretty much 100% this cannot be realed to the water n thermostat change. I have just replaced some of the vac pipes as I know this can cause this affect but it's still doing it. The hot running still seems to be happening too but this could be a faulty temp sender. Any ideas on either problem would be greatly appreciated. Chewy