Constant freezing

I have a rooted Lenovo Yoga 10+ HD, which a while back started freezing after only a few minutes of use. I performed a factory reset - wiped the lot, and started from scratch, installing apps as I needed them, and trying to be a bit more careful and selective.

And for about a week it worked fine. But just today it's started freezing again. I do a hard reboot, it starts, and with a minute or two it locks up to the extent that nothing but a hard reboot will unfreeze it. But having only two minutes (if that) of use means the tablet is effectively unusable. I don't want to do another factory reset - can anybody recommend what might be the problem? I have the most recent version of Android for this tablet (4.4.2), and if I select "System Update" I'm told that I do indeed have the most recent.

I suppose I could unroot it (if I can have the tablet working long enough to download and install an unrooting app); but as it was working for months happily rooted earlier, I'm doubtful that this is the problem.

Anyway, I'd be grateful of any advice!

Thanks,
-A.