i had the exact problem on my 2wd saph...... if you look at the front of the car at the engine, look at the plenum chamber, at the very front of it there is a 10mm bolt that i think holds the fuel rail on or the accelerator bracket. the ford ecu loom rounds against it, and some times can wair through the loom, mine done this and sometimes run under 650rpm on idle sometimes was ok though. after some time it rubbed through the coil ign live wire, which then blew my ecu up!!!!!!!!!!!!!!!!! brought a new one, then replaced the hole ecu loom with a motorsport wiring solutions loom (john does all the ford rallye cars looms) he is the man for these sort of probs. 07802 808751.