iCal hanging importing ics files into iCloud calendars

After recently enabling iCloud on all my devices, I saw a lot of duplicate events in iCal.

So following advice in "Take Control of iCloud" from Tidbits, and this knowledge base article:http://support.apple.com/kb/HT4967

1 - exported all my calendars on the machine that had the most complete calendars (my Mac Pro),

2 - deleted all calendars on all my devices,

3 - and am trying to re-import the calendars I exported in step 1 into iCloud calendars in iCal on my Mac Pro.

But on many of my import attempts, iCal is hanging in the "Importing iCal Data" dialog, never finishing. Eventually I end up killing iCal, then restarting it. At that point the calendar name is there, but I'm not sure if it imported successfully or not (in some cases, I don't know of upcoming events in that calendar that I can check to see if they appear in my calendar).

Any ideas? I have a Mac Pro and MacBook, both with Lion 10.7.3, an iPhone 4 and an iPad 1, both with iOS 5.0.1.

(In just one case, a calendar seemed to import fine without hanging at all, but then events that I know are in that calendar didn't appear in iCal).