Drive write efficiency during gaming capture, esp at high res in games that are high resource use, is crucial to getting stable frame rates. This is why I prefer DxTory, because it can use multiple drives to strip the file in parts and write them simultaneously.
Fraps has evolved slightly to try to stay in step, but even it's default option to leave framerates unlocked is not what many think it is. It only avoids locking to received frames. Some have implied due to this setting Fraps can record at say 30 FPS while allowing the game to play at 60 FPS or higher, but this is not generally the case.
"Contrary to what the name of this setting implies, this does not lock the frame rate to the actual recording rate specified. This setting locks your frame rate to the actual rate of received frames. For example, if you have your game set to record at 30 fps and during a system intensive moment Fraps only receives 15 of those 30 frames, your frame rate within the game would drop to 15 fps."
http://www.ehow.com/how_12195700_use-fraps-loss-fps.html
What this means is you can set Fraps to avoid framerates below the capture FPS setting you select, but it won't let the game play at it's true unbridled FPS like DxTory can. I guess a more accurate word for game capture framerate limitation would be hindrance, vs lag. Many are used to thinking capture lag means framerates below your capture setting. Those with high end systems compromise by setting FPS to 60 and put up with huge capture files and long editing sessions. The best scenario is being able to record at 30 FPS, which is fine for video playback, while being able to play at the game's full framerate.
I've captured Metro 2033 at 720p with a mere GTS 250 with plenty playable framerates and max texture detail. I always use 30 FPS, and in Dead Island I was getting 100-120 FPS in game while recording. With my current 7970 there is no game I can't easily record at 1080p, and I still use just 30 FPS capture rate.