I found this post from Sir Ben if it helps.
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I've recently been through a similar saga with my '99 DSE. Driven locally it wouldn't overheat at all, and it was just running much too well to be a head gasket gone, but put it on a dual carriageway at 70-ish and after around 20 minutes the temp gauge would ping over to the stop and it would go into limp-home 5 mph mode. Really couldn't believe it was problems with the head and the rad seemed to be pretty warm all over and showing no blockages, but after changing thermostat, pressure cap and temp sender, I bit the bullet and had the head off to find no sign of gasket problems. Had to have the head skimmed and new gaskets, then off up the dual carriageway and back to the overheating after 20 minutes of higher revs, just like before.
Then I heard about problems with P38 radiators. Apparently there is some sort of baffle in the rad that sends the water right down and around the whole radiator to keep things cool, but if this baffle fails for any reason, the water goes in through one port at the top of the rad, straight across the top and out of the other port, also on the top, without passing through the whole cooling matrix. The rad still warms up reasonably well, but doesn't do much cooling and after 20 minutes or so the temperature creeps up and it overheats.
It sort of made sense to me, and by this time I'd spent a small fortune on the engine with no success, so I put a new radiator in and have had no problems since. The funny thing was that I'd thought the rad was blocking and was disappointed when I put water in one end to flush it through, only to find that it was flowing freely out of the other end, so I wasn't too hopeful at that point. Perhaps it's something to do with both hoses being on the top, instead of one at the top and the other at the bottom like in most rads. But it did the trick and now pulls as well as ever.
Anyway, I hope this may help others trace their heating problems.