After fiddlin' about with this, I think I have discovered the solution and though I would post it in case anyone else has the same problem.
One of the two computers I sync with was set to synchronize 'future' or 'pending' items in the calendar and task list. The other was set to sync 'all'. Depending on which computer I synced last, this could cause some strange goings on.
I have changed the one to "all" in the advanced config settings and this appears to have solved the problem.
Do I get a prize for solving my own problem?