check in order;
fans are coming on(bridge them make sure they work)
fans are blowing right way(make sure its blowing and not sucking)
blockages in the rad(give it s aflush out)
thermostat(prob best sticking a new one in anyway,only pennies)
last one head gasket(felpro are not the best gasket for a rs turbo,especially a tuned one)

also how hot does it get(forget he gauge they lie ) is it bubbling over or is hoses gettin pressurised badly,the front mount will restrict air around the rad but 2 fans should bring it down i would reckon,fact you mention it goes down when movin prob points towards the ic infront of it,as long as its not boiling over it should be ok