Intermittent overheating only under load

I've got an mg zr with a 1.8t in it. I recently changed the radiator as the old one was slightly leaking. After fitting I topped up with coolant and bled system using the bleed screw. Been driving for a week with no problems to and from work keeping my diagnostic machine plugged in and keeping an eye on temp, the temp sits anywhere between 90 to 94 degrees and if sat in traffic will rise up but always cools down when I get moving.

Anyway I went on the duel carriageway the weekend cruising at 70 for roughly 5 mins and I could smell coolant and the thermostat on my dash rose up to three quarters so I pulled over and coolant was boiling out of the expansion tank I turned the engine off and let it cool down and got home using normal roads and it was ok after that. Any idea why this happened?

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