I had the same issue a couple years back but i found out what it was shortly after. I was doing work on installing a new radiator and i wired up the fan the wrong way so it spun in the opposite direction so when there was enough wind force the fan and the wind would cancel each other out but it would only happen on highways/freeways when doing 90-100. I doubt your having the same problem but i thought i would put in my two cents. Outerwise get your garden hose and undo the top radiator tube and just give it a good flush for 5 mins and if you've already had a service on it two years ago then get a new thermostat.

Otherwise dude your welcome to borrow my stock radiator to try and resolve the problem. Your in perth hey.