As above really. Could a slight air leak in manifold cause it to run fine at idle and fast idle but a missfire in the upper rev range?

If the misfire is worse under load check the ignition, plugs, plug leads & coil packs also LT ignition wiring.

Yes it can, especially with an injection set up, sensors get the wrong signals and the ECU does the wrong thing.

Having run a lot of jap cars over the years, a few developed under load high rev misfires, always turned out out to be coil packs, I changed the coil packs, problem solved, I may have been lucky, but with a misfire I would first start with plugs, leads, coil packs in that order, it might also be worth checking fuel pressure, pump and regulator and injectors.

[Edited on 19/05/04 by PAUL FISHER]

Have you sprayed wd 40 around the inlet this will generally cause the revs to rise if a leak is detected you can has a colours gas / smoke sprayed around the induction side if a leak is present it will show from the exhaust exit.

Brother in laws 200bhp Zetec developed a misfire under load did every thing to it plugs, leads coil pack injectors rewires ne pressure regulator, pump it turned out to be his swirl pot which was too small He has since fitted a Bigger swirl pot / seperate item and a low pressure pump in line problem solved.

I'm just curious as the co emmissions are also slightly out at 0.29 at fast idle so thinking this is causing the multiple faults?

Is it your inlet manifold thats leaking air? if it is, then fix it first, then you have ruled it out, but that could cause high CO, at fast idle, the extra air could be telling the ecu to feed more fuel, but it could also be, a faulty lambda, cat not getting hot enough, a faulty fuel injector, this could also be your high rpm misfire

problem is the s2000 coilpacks are the built in ones and cost a fortune!
its not the inlet manifoild its the exhaust manifold i think