MG TF potential thermostat issue?

Hi i have an 03 mg tf, in january it had some work done to it and apparently it didnt have a thermostat when the guy did the work on the head gasket, so it had a new head gasket, cambelt, alternator, alternator belt aswell as a thermostat put on it, the issue i am having is that when on the cold setting, or anything not on full heat, the water tempreture rises and as soon as you put it on hot it drops instently to under half. when just idle and then giving it a few revs on lower heat it gets to 3/4 temp on the gauge and drops to below half, any advice on potential issues? should i remove the thermostat again? (the radiator isn't blocked due to me checking that just the other weekend and it was clear)

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