I ran 5.3.1 under Mac OS X 10.4 for a long time without any problems with QD/QC. Try repairing permissions if you haven't already. If that doesn't help, you might try uninstalling NUDC (using the Uninstall script in the Extras folder) and reinstalling and see if that makes a difference. Back up your calendar and contact files, of course, just in case something goes amiss.
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Dave Sawyer
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iMac 24 2.8GHz Mac OS X 10.6.2
PowerMac G5 2GHz DP Mac OS X 10.5.8
Windows XP, Vista and Windows 7