exhaust manifold glowing

hi ive recently replaced the head gasket on a 52 plate 1800 petrol after having the head skimmed and pressure test (everything ok) but on reving the engine in neautral between 3500 and 4000 revs the exhaust manifold starts to glow a bright red.Let the vichicle idle and evrything is fine,its been road tested and drives fine with no lack of power and will idle alday smoothly starts first time everytime.Can anybody help please

They all do that!

It wont happen when you drive as cool air will be passing over it

hi thanks for your reply
have you ever seen this happen as everything above and below it gets red hot to, is it only freelanders that do this as ive been a mechanic for 21 years and have only come across it once on an astra and the ecu was at fault.thanks