Isync Fails On One Computer


I have three computers, a G% and two G4s, the G5 DP 2Ghz and G4 single 500Mhz run OSX 10.3.9. but the G4 DP 500Mhz runs 10.3.4. (reasons of Music apps) i keep in sync with iSync. The system has worked well for over a year.
Suddenly the G4 single started to fail, part way through the "Downloading changes" steps which is the next one after the "Checking and Optimising" steps, the failure would always cause SyncServer to "Unexpectedly Quit". There is no suggestion as to what is causing the failure.
Does anyone have some suggestions as to how I might correct this problem?