Yeah the fans seem to be running fine. Surely old mate at Natrad would have taken any opportunity he could to get more money out of me though if the radiator was a bit suspect? It'd really suck if it was the radiator because just 2 weeks ago I left a job where we supplied spares to smash repairers (incl. radiators) and I could have gotten it really cheap.

It seems to just do it after 30+ mins of driving, either in traffic, on the highway, or a combination of the two. Thankfully it's not occurring too often and I'm aware of the risks so on a few occasions I've had to stop for 20 mins to let it cool (making a  45 min trip take over an hour). It's not leaking/losing coolant either, just overheating. It's done 160k kms so I've still got a bit to go before looking at the timing belt. I'm new to these engines (I've only ever worked on old Toyota and Holden engines, none of this fangled computer stuff!) so some help in finding the thermostat would be helpful.

*edit* It's probably actually just my experience with these older engines that made me suspect the water pump to be honest. I don't really have much else to base it on

As said above air will cause overheating issues. Have you recently changed coolant. I'd try bleeding the system  (facing the car up hill sometimes helps) then the cap, then the thermostat. I think on your motor if you drain coolant and take off the bottom radiator house (engine end) you should find the thermostat, that's also the base of the water pump.
If they don't work then you may have bigger problems. The fact its overheating while on the free way is a worry, in fifth doing 100 there should be ample water flow and airflow through the radiator.

How is the overflow bottle looking? does it fill up every time it overheats?

is there any black sooty coloured stuff on the bottle (not necessarily floating or anything)

go to the mechanic and get a TK test done. it checks for exhaust gasses in the coolant.

this is known to be an issue usually with EJ25D engines (gen2 rx,heritage, outback with quad cam) but it has been known to affect a few early gen3's although slightly different it could be a head gasket situation.

i dont know about anyone else but mine would heat up in traffic and if i accelerated hard it would spike within a minute but on the highway (not climbing hills) it would be ok unless i was low on coolant)
as far as the thermostat goes, only go OEM imo, most are designed to fail open but there are a few cheap nasty ones that dont.

I wouldn't say the DOHC's are prone to cracking.. in all the ones I have done its always been a leak on the passenger side exhaust leaking into coolant and let me tell you one of those blocks was well and truly COOKED! and the heads still were fine, you can clearly see where the gasket has failed. this is due to the shitty factory gasket they used.. they revised the gasket to a MLS (multi layer steel) gasket and that fixes all the problems there.

the SOHC get a external leak usually, this is usually pretty easy to tell from the tell-tale white stains on the bottom of the block where the coolant drips and dries.

usually its not too bad as long as you keep the coolant up.
but it is still possible there is a internal as with the DOHC's

but lets not get the OP panicking lol, it could still be a thermostat (and if your are smart you will collect all the fresh coolant into a CLEAN bucket and re-use it. it's really easy to change the thermostat.

bottom radiator hose connects to the thermostat housing, remove hose from thermostat side.
remove thermostat housing.... it is 2x 10MM (maybe 8mm) bolts, they are quite long though. DONT round them! slightly tighten then loosen them, because of where they sit it can get a bit dirty there and they tend to not want to come out easily at first. once that is off you will see the thermostat, pull it out, (TAKE NOTE OF THE POSITION OF THE HOLE ON THE OUTER RIM, this allows small bubbles to pass through and also lets the water flow a bit when the thermostat is not open (only tiny amount i think its just for when the car is cooling down so that it does not build neg pressure)) if it goes in the wrong way bubbles can sit there (well thats what i was told)

if you are doing this yourself i would hazard a guess that you know what to do from here.

Well, courtesy of a death in the family and a pressing need for me to travel the 750km to Coonabarabran tomorrow morning, I brought my weekend project forward and did it thisafternoon. It was remarkably easy. I really wish I had remembered to do a photo tutorial. Anyway, I guess I'll find out tomorrow morning at about 3 whether it was the thermostat or not: stay tuned!

Can you tell me which way the thermostat goes. Its the only think I didn't note before pulling the engine apart. I know top from bottom obviously, I just need to know the orientation. Cheers