It really was go back to basics . We had tried to resolve the problem based on a series of tests based on the information we could get and with no stored codes and all live data looking ok we were pretty sure it was 'mechanical' rather than any software or electrical issue , when the problem was still evident following ECU change we decided to just go back to basics and look at the sequence of events that unfolded from the start .We had a misfire and on compression test we found a valve problem on cyl 1 , head off and sent out to engine shop for repair and all seats cut and valves refaced , head refitted and then the problem appeared after test run , replicated after 17 mins of idle , anyway following lots of replacement sensors etc problem still existed and this led us to firstly check all the valve train for damage including all rockers and vanos units concentric shaft etc etc all with no resolution to the issue , decision made to just swap out the complete head and hey-presto problem solved . a quick check on cyl 2 exhaust valves show a very tiny area where we had 'seal' just enough to cause no problem when cold and major problems when hot btw compression tests were all pretty equal and around 170 psi . we will do a more comprehensive post mortem on the head when we get some time . but I think it was a case of assuming everything was ok with the repaired head and getting bogged down on possible engine management issues when we should have ruled out the basics first .
G