Another p38 problem 4.0 auto Vreg

hi everyone i hope somebody can help as i am at the end of my teather,its close to a box of matches ? as with most p38,s ,overheated!cylinder heads removed skimmed and presure tested,6 of the cylinders allready had replacement liners fitted,replaced with multilayer gaskets,seals etc,started the car the overheating seems to have inproved but car did not perform properly,read engine fault codes,numerous faults,po174,po171,po130,po327,cleared and road tested and reread,po134a,po327,replaced both oxygen sensors and n/s knock sensor,road tested and found it had no power,read engine management fault code memory found it was clear of codes.went for m.o.t.failed on emmisions,rechecked fault memory code,still nothing stored,suspected a faulty idle control valve,got a used one passed m.o.t.(Summary)...fault code clear...passed m.o.t.,,,all cylinders have correct compression,,,the misfire summary is and as always is zero,,,fuel pressure is within specifications/tank pump brand new,,,the catalyst assy has been changed,,,both knock sensors new,,,oxygen sensers are new,,,idle control valve been replaced,,,engine ecu has been swapped with another car,the fault didn,t transfer to the other car??,,,air flow tried from another car...no air / exhaust leaks,,starts first time every time,,ticks over smoothly.Please some body come up with something this has been ongoing 8 mnths??

its a long shot but cats might be partially clogged up?

we changed the front twin pipes as we thought the cats might be blocked as well.neil