here is a FIX for this issue:
the problem is not the n900 or PC Suite it is that your Outlook calender folder has entrys which corrupt the sync process.

I think the cause in my case was on the N900 I merged - skype contacts into the original outlook sync contacts - which is fine - but skype contacts have 'birthdays' and if you travel to differnet time zones it is this data that starts to cause things to get messed up.

Future prevention : update the outlook sync contact on your N900 with the skype address only - dont use 'merge'


Now to fix the calender Sync: what I did in Outlook was
1) copy the complete calander to the archive folder:
2) use advanced find selectind folder calender to show the complete list of entrys - CTRL A select all then delete the entire contents of the calender.
3) click to veiw both the personal folders calender and the archive folders calender ( note there will be two in here now) one is youe copy ... then selct all the entrys moving forward from today and 'MOVE' to the original now empty calender folder

in PC Sync - in the wrench settings - change to
1)just calender selected
2) direction.>just Outollk date to phone
3) >delete content on mobile phone and

Now when you sync - it may take 30 seconds for the phone to clear itsself but you are back in business

after this first process : reset the Pcsync to all your normal settings