Can't do anything except paste text

I am having a weird issue with Storyline 2 where every now and then, as I'm adding slides, I suddenly can't add new ones, can't copy old ones, use Duplicate, import graphics or movies, and so on. I can do a few things, like add shapes, delete or edit existing objects, but every time I try to do one of the things above, Storyline just pastes the last text that I copied into the current slide as a new object. This happens even in Story view, which is particularly confusing.

I have tried restarting SL, rebooting, and even reinstalling. Sometimes I'll get through 20+ slides before it seizes up, other times I'll be doing them one at a time and having to constantly restart just to add the next.

It DOES seem to work if I copy my text out of a text editor instead of Word (yes, I use paste as text, so this shouldn't matter). But, I don't trust it working for a few slides and then not breaking on me again later, even with that method.

This is incredibly frustrating -- any suggestions?