assuming the t'stat is working, removing is not a good idea as it regulates flow and controls the coolant temp better than just allowing it to circulate regardless of temp. also takes ages to warm up.

id go round all the hose clamps again. do your mates with rst' have std header tanks or the fabbed alloy ones. im srarting to see a pattern where cars with alloy header tanks seem to bubble. still, could be a stubborn air pocket.

id be suprised if its a hg.