So normally I can pick locks just fine.

But just occasionally I get one that really, really, REALLY slows up... or doesn't allow me to pick at all. Right now I'm stuck in Caduca's chambers and the game won't let me unlock the chest in her side room for the last piece of loot in the level (though weirdly enough, I was somehow able to pick up a piece of loot which was technically IN the locked chest, that I never saw, I only discovered what had happened when looking at the spoiler walkthrough - go figure! Somehow I picked it up through the wall of the locked chest)

I have read that it's something to do with limiting frame rate. And that frame rate was also supposed to be the major cause of slow-down issues in some other areas which I had serious trouble with when I first played the game, years ago... but I had no problem at all with it today (the most egregious such area was the Abysmal Gale ship, which played entirely normally for me this time through.)

So. What's going wrong, if it is a frame rate issue, is there a way to limit frame rate WITHOUT needing to download a third party piece of software, or indeed WITHOUT having to download Thief 3's "Sneaky Upgrade" which appears to be not just a bug fix but install some actual changed behaviour that I do not want?

Update on this: It seems to be location-specific, as it was before. Specifically, this time, to the Lower Libraries of the Keeper Compound - Caduca's Chambers and the Hall of Statues. The main Keeper Compound is fine, the lockpicking game worked perfectly. But as soon as I got to the Lower Libraries... well one chest won't start the lockpicking mini-game at all, a second one seems to have developed the same problem even though it worked yesterday, and the third seems to only allow me to start lockpicking if the nearby torch on the wall is lit, otherwise Garret just waves his hands for about 5-10 seconds and then goes back to wielding his weapon.