Sharp drop in FPS while recording
Most screen capture applications are pulling video from the video card down to main memory to compress it and write it to disk. Most video games are uploading textures and geometry to the video card to display it to you. It's possible to get into a situation where more memory is needed on the card to run the game and record the video than you have available. That's usually when you see frames dropping. Depending on the game, it might happen when more characters or scenery is visible (such as when viewing a scene from high up, and it's all in frame). The memory on the card is limited, and the ability to move data around in the computer is also limited. It sounds like you're hitting those limits.