I have never come across this problem before. I've seen it at the start of some replays but never happening part way through a replay.
Sounds like something more serious too me. Have you tried running the replay after a fresh PC boot, or after closing most of your background services etc. Try lowering the resolution, or even try running in Windowed mode (or full screen if that is opposite to what you currently run).
Have you got Camhack running at the same time, if so, try turning it off for the replay.
I'm really guessing here, but all easy things to try.