I got drops during the oil refinery mission also, but at the beginning. For some reason, the small cutscene that showed BB jumping out of the helicopter gave me single digit frames, with GPU usage dropping heavily for no apparent reason. After that, when I was going through that actual area, I wasn't getting drops below 55-60, but I was getting very high GPU usage.
As others have mentioned on this thread, there are a few cutscenes that plain old wig out, and, again, for no apparent reason. Along with the aforementioned, I had this issue for a cutscene during the prologue (don't remember which), the cutscenes after extracting Code Talker, and the cutscene that takes place when Battle Gear is finished.
A couple have also suggested that HBAO+ is a factor, but then I've also read that some with in-game AO experienced the same issue. I have yet to get an S rank for the Code Talker mission, so I may go back soon, beat it, and see if the same thing happens again.