sounds to me like you have a stretched chain, hence the timing being retarded. on later models eg om606xxx the ip drive gear has an advance/retard mechanism in it, if yours has this arrangement then it could well be worn slightly. the mechanism works by centrifugal force, so the faster the engine speed the more advance for the injection timing. to be certain as to the correct diagnosis i would recommend you have the chain checked for stretch, there have been numerous posts regarding checking chain stretch.
regards
wagger