Background render bug - freezes machine when trying to use it

I've seen other reports of this exact issue in the past, though few and far between, e.g. https://discussions.apple.com/thread/4890706

I've been using FCPX without trouble for months, but just yesterday background rendering started causing an issue. I've set it to 5 seconds, but if I let it kick in by not moving the mouse, it won't stop rendering when I do come back to the machine and move the mouse.

This freezes the entire machine for 10 second jumps at a time. I can click something but it won't respond for 10 seconds, then I get a little bit of response, then it freezes again for 10 seconds. Machine unusable.

I turned off bg render and everything works beautifully. But I like that feature! I'd like on as designed.

I've done a hard reboot and the usual things.

Is there anything else I can try? Some sort of repair function? Though I believe this is a bug.

I have the latest updates 10.4.5.

Thanks.