We've seen this same issue since upgrading to Creo 8, and actually have a ticket in about it since it's reproducible.
It was happening randomly for our users, but was across divisions and environments, so we don't think it's a config option.
It also didn't produce any crash reports or actually crash, it just sat there like it was waiting for a window prompt.
The issue we've noticed is the "undocking" of panels in the Symbol section of Creo 8 causes them to not appear the next time the symbol is selected, therefore locking the session of Creo.
For example, if you drag the symbol customization panel off the ribbon, then close the drawing and re-open it with that setting, it freezes.
Anytime there is another window that pops up (like when working offline, license notifications, or the clock window) the symbol panels appear correctly.
Deleting the creo_parametric_customization.ui file (where those panel positions are stored) resets it and allows you to select symbols without it freezing.
Alternatively, using the Reset Panels command in the "Windows Settings" of options resets them from within Creo.
As soon as those panels are undocked again though, it goes back to being locked the next time a drawing is opened.