How old is your rig?

[Pete Grey]

One british vehicle with a habit of boiling up was the MGF, the recommendation was to remove the thermostat jiggle pin and drill the hole a little larger. This allows some coolant flow even if the thermostat fails to open.

You should be ok with a new thermostat, i bet you tested it in boiling water first, i always do.

If the problem continues you could flush the whole cooling system, i use a coolant called 4life which boils at 180C (350F).

I used it first in my MG midget and now in the BOV. It also gives frost protection to -40 plenty good enough for UK winters.