hi m8, im having exactly the same issues at the mo (see my thread, cossie boiling water), it drives fine then all of a sudden, if sitting in traffic for more than a min, loads of steam comes out of the bonnet vent. almost look like its on fire!!!!

mine is also hunting when its warm. infact i have all the same symptoms as you.

done some checks, head gasket seems fine, and no traces of oil in water or white mayo anywhere in engine. thought it was my fans not cutting in in traffic, but my fans are working. you can test this buy putting the air con on, they should come on about 30 secs later.

i too took of the cap and couldnt see fook all there was so much steam.

had a good poke around underneath the return rad hose was rock hard with pressure and v hot. felt the inflow rad pipe and it was barley warm.

have to felt the hoses for temp difference.

im just letting the old girl cool off then im gna whip out the thermostat, and give it a go.

i cant believe we are having identical problems, thought i was the only one. only had the car 5 days!!!! but it has been sitting for 18months. so its bound to be a bit stiff.

i'll let you know how i get on. have you tried your stat yet??