I would tend to think that its not cooling system related as when you are traveling at higher speeds that is creating maxium air flow for the radiator......
Possably incorrect spark plugs, check ignition timing, as this could be causing too much heat in the chambers and could cause huge dramas!
Good luck with this keep us posted on your findings.

Maybe the problem is not in the cooling system because in that case the overheating would be more severe when driving slowly, I think. Could be a lean mixture which causes excess heat when working the engine harder. Whatever it is, take care of it or bigger repairs are certainly ahead.

Is it a twin core or a tripple core radiator?
twin core might be the problem.Was my problem.I replaced with a tripple core.It is now fine.Replace thermostat also.Might be old.Might also be waterpump.

Doubt it's the cover under engine that you are talking about as I tested mine with and without it. No difference in temp.

nah not modified, just a stock a12gx, stock rad, stock fan.
the rad was only reco'd 10000km ago, but that was 4? years ago. the only thing i havent put on there yet is the steel guard underneath.

yeah either dodgy radiator, or more likely a non proper shrouding of the radiator ie cant driect the airflow through the radiator properly. im guessing its been modified. thermos and stuff?

sounds like when the going gets tough..the tough stop working..

Sounds like a ####ty radiator.

few teething issues im trying to get sorted on my ute.
driving round town, no dramas, temp needle sits about halfway, but as soon as i get on the open road(80ish kph and above) it starts to run a bit hot. ie. just below the 1st hot line.
is this a classic case of 'forgot to fit the stone guard thingy underneath'?