When I had a misfire on mine, it was only "foot to the floor" when it would happen, normal driving it was fine. Fitted new plugs and it improved a bit, then I fitted new HT leads and this solved it. The MY99 onwards cars have the leads running pretty much right over the exhaust rather than the separate coil packs on the earlier cars, and as such the leads are more prone to breaking down.
Hope you get it sorted out.