Hi guys I need some advice,

I've got a Mini Cooper Knightsbridge year 2000 1275cc Mpi engine but converted to spi. Ive got an issue with the radiator expansion tank coolant boiling when I come to a stop , but when I start the engine back, it stops boiling. Temp Gauge never goes above half.
I've check the gauge it works fine, Engine does not overheat how much I drive.
I have cleaned the rad, fan works fine, was not sure if it was the thermostat valve so I got it removed, now I'm running without it. but still the problem remains the same. Is a thermostat valve necessary ? what could be the issue?