I played this game up until the Watergate chapter where you have to sneak past these two painters to reach the door at the end of the hallway. I watched a video walkthrough of this and in it, the painters are supposed to move, with the one on the left going through the door. But in the GOG version, they do not move at all and glitch every five seconds.

Weird. I'm playing the GOG version and they moved. I had a problem later in that level where I didn't trigger the guy on the radio telling me where to go and then he'd say "sorry chico", and plop I'd die. I restarted a few times and it finally worked. I think events are triggered at very specific points in space. Try moving around to slightly different locations.

Remember to quicksave often.

I think I'm at the "2nd phase" of this ridiculously difficult sneaking-through-the-Watergate level. I'm at this point where there is a ladder and a guard standing on a stairwell. The guard doesn't move at all and whenever I go a different direction the player dies. If I try to move past the guard the player dies as well. Is this a glitch?

Update: I think I finally figured it out. It looks like I went through a wrong door and the game didn't script properly.