Radiator cap would be a natural to replace and check it out.

If this doesn't fix it, there could still be an internal engine problem...
A new headgasket was a bold move, but might not have gone far enough. Think
about this...not all internal engine leaks allow coolant into the cylinder.
A crack or leak can sometimes allow blazing hot exhaust gases into the
coolant jackets. A couple thousand degrees of gas temperature going into
the coolant at high pressure can blow past any radiator cap, bubble, and
cause extreme overheating. Not the usual case, but it happens.