I had an issue a LONG time ago similar to that with imports. Would let me work on the item for a bit them would just stop, not lock up or anything, just wasn't able to do any work on the object anymore. Only did it on certain items, think they were off some Magazines content CD Found if it's an issue I'd undo a step or 3 of my last edits, export the item as a .obj or 3ds and re import. no more issue... had something to do with the file format or some such thing.. really no clue, but it worked.

hope that helps, if not... no clue  :(

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Hexagon, Carrara, Sculptris, and recently Sketchup.