Well I checked the fuel tank for water, no water present. Removed the evap valve from the intake manifold and put a vacuum gauge in its place and still we had the same problem of low manifold vacuum at idle and P0300 and P0301 I noticed that if I raised the engine speed with the throttle slightly the vacuum increased to a more normal level so checked the specs to find the idle speed is to be between 730 & 900 rpm our was set to 730rpm so re-programmed to 850 and hey presto normal manifold vacuum and nice even idle, no more trouble codes and passes cat test no probs There is still the occasional uneven beat from the exhaust but doesn't seem to be detrimental to the running so I'm not 100 per cent convinced. Only time will tell?