A common problem or so it seems!
When mine did this it was a dead ISCV, replaced it twice, once with a supposed brand new ISCV and I still had idle problems, replaced it with a second hand item and it's pretty sweet and well behaved now!
Where did you get your brand new ISCV from as they are no longer made I don't know of anyone who stocks them anymore!?
burtons about year ago they dont do them now but they still do the escort cossy do you know if they are the same iv tryed about 4 iscv and still the same