Sudden Problems with GOTY on PC

I have been playing GOTY since it was released for just short of 200 hours. I recently got a new desktop PC and have played a good few hours on that. All of this with no problems.

However, yesterday I started noticing screen tearing. I also noticed that my character is “pulling to the right” when I press the forward (W) key, so I’m constantly compensating with the left (A) key. I’ve used my keyboard, keypad and mouse for “forward” and the problem is there on all of them, so it doesn’t seem to be a hardware issue. I’ve tried other games, including BL2, and there’s no problem.

Is anyone else suffering this? Steam doesn’t list any recent patches. Any help would be gratefully received.

EDIT: I verified the installation, which came back 100% OK. However, going back into the game, everything was back to normal! Given that I had the problem last night and again this morning after a shutdown and restart, it doesn’t strike me as a memory leak. More puzzling is that I didn’t restart after the verification and everything was fine. I no longer have a problem but if anyone can explain this, I’d be very interested. Ta