Renault Megane Sport 225 dilemma?

Hi, I have a 2004 Renault Sport Megane 225 Phase II. No mods, K&N short ram replacing the standard air box setup. Starts first time every time but the  idle is lumpy & struggles to get going running on 3 cylinders, once over 2500 revs back on all four and were off like a scalded cat? With Shorts bursts of throttle boosting then back off totally it has a misfire (over/fuelling) backfire. It has a whistle from some where but not a turbo sound more like a constant kettle boiling, when on throttle boost comes on strong with the induction noise more like 100 people telling you to shoosh. Temp gauge reads constant when warm, it does surge a little say at 120kph light throttle kind off like the hand brakes on then let off for a second & back on...? When coasting wont like being in 5th gear at 80kph the intake manifold shudders, feels like your running over cobblestones. I have checked coils all good, new plugs no fouling, cleaned TDC sensor, cleaned MAF sensor, checked all piping for boost leaks, checked all vacuum lines for cracks, cleaned throttle body & stared into engine bay intensely for hours...? It has no fault codes or warning lights ablaze, all thats left is to clean the egr wherever that is or hire more people to stare at it on a roster system I guess. Any ideas would be greatly appreciated. Cheers .

Graham.
February 2013