well as the title suggests the coolant system is pressurizing and when on boost the temp gauge drops, i think it could be the headgasket but could it be anything else!!!
id replace the thermo then mate ,sounds like it is getting stuck open,causing the needle to go cold ,if it was headgasket you would be over heating and needle should rise,anotoher sign of headgasket going ,is a milky resude under the oil cap,or water been pushed out the over flow bottle,thermo is cheap and really easy to change,should fix your problem ,guide below