hi there

THe mods you metntion wontmake any difference, the zetec already has platinum plugs and the leads from motorcraft are ideal, any other leads can cause misfiring! Induction kit, unless a well known manufacturer may not last very long and may give you less power!

As far as the problem goes, it sounds like speed sensor fromt he gear box, so when you are stood still it idles at 900rpm, but if you were to creep down a hill the revs would climb to 1200? SOunds like it s this. If you look at the rear of the gear box near the speedo drive there should be a wire coming up to a connector, tyr cleaning this connector!