How to I achieve a constant Framerate on Videogame "Demo Playback"?

Alright, so, here's a little backstory: I am trying to use Serious Sam HD 1 and 2 to make a fan trailer advertising the two games. I am using demo playback and FRAPS. The demo playback allows you to play a game in with any graphics setting you like, and then play it back with completely different settings if you please. So that is what I am trying to do: Play in Low, the Record in Ultra High. When I am viewing the recording, it plays back at around 0.5-3 FPS. Because of this, when I edit it in Sony Vegas to play 60 times as fast (to get 60FPS at the times it records 1 FPS), it sometimes goes slightly faster and slightly slower in certain places.

I have seen videos in Crysis where a similar problem was encountered, and the guy was able to fix it (Video: http://www.youtube.com/watch?v=YG5qDeWHNmk), but didn't explain how he got it constant. Can someone please tell me how i'd do this in Serious Sam HD 1 and 2? It will be much appreciated.