idle and constant speed problem getting worse

[Ed Doe]

Yeah but have you changed the plugs since changing the fpr? If you sooted up the plugs by dousing them in fuel, they wont be as good as new ones now you've sorted that issue...

Do you have a non-oem exhaust on it? If so is it properly loud? If so the 'missing' is highly likely to be down to the reduction in back-pressure causing mild pops and coughs.... which is entirely normal! My old '6 felt very jerky at low rpms on light throttle when I had my old exhaust on it cause it was a proper straigh-through with very little baffling...