I did more playing around with the Q5 Pro this weekend. When I set the Q5 to 1080p instead of 4K, I can watch and stop any video and I don't lose video signal. The only problem with this is that 4K videos get scaled to 1080p as well. I set it back to 4K, and when I stop playing a video I lose signal again.
So I installed the auto frame rate/res patch to see if it helped. Sure enough, it did! I can watch any video, stop it, and select another video without losing signal and having to reboot every time. Awesome! It's back to the way it should be. Why that fixed the problem, and why it happened in the first place, I have no idea. And I don't really care I suppose lol, since it's working the way it should!