B1981ci

I never owed a car that would overheat just because you drive fast and I've probably owned 20 or 30 different cars. Some new, some old and high mileage, economy cars, sports cars, etc. None of gthem would overheat just because you drive on the highway at, let's say 80% of the red line. I also never owned a french car or a turbo diesel so I just wanted ot know if it's "nomral" or if anybody knows what can be the cause. I am curious though why if it's crap in the cooling system does it not cause trouble at slow speeds?

It is an "xsara break (n2)" by the way. I guess if this info is accurate then it must be a 1.9 or 2.0 not a 1.5... http://www.auto-data.net/en/?f=showSubModel&modeli_id=1693

The car overheats rather quickly bit only at high speed on the highway (160-180+ kmh). This is the only time it does it though. Drive below 160 and it's fine. I thought maybe it could be the thermostat but if that were the case I would expect problems at any speed. Car otherwise runs smooth, no noises, no lack of power, fuel mileage is great, etc. What could be the cause of this and how do I if it. I think the engine is a 1.5L turbo diesel but I'm not sure (it's definitely a 4 cyl turbo diesel). Thanks!