I have the same problem. Mine has different trigger than yours.
It was triggered by clicking network indicator then the UI freeze
exactly like yours.
I installed Luna 32bit in very old system:
AMD Sempron(tm) Processor 2800+
RAM 1.3 GB
VGA GeForce 9800 GT
Motherboard NFORCE4-A754
And I filed bug report some times ago:
https://bugs.launchpad.net/elementaryos/+bug/1097604
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