Got a bit of a weird one at work, engine on a dyno, using thermostats to control engine temp through water radiator (normal setup). Engine is a 7.5 litre diesel, runs fine up to 2000rpm but above that overheats?? Water temp across system is 20 degrees top vs bottom at low speed, but at high speed they seem to converge to within 5-6 degrees and then the water temps go above 95 and we need to shut it down? Any ideas?