Sometimes, not always, when I download a movie from iplayer the sound does not sync with the actors lips.

Also, I can't always fast forward or "jump" from one spot to the next. I can only fast forward 2x instead of 32x. I think it may only be a problem with new films?

The FLV format is rather brittle. I'm not sure if it helps but you could try another player (such as VLC), or you could try an FLV fixer program (I don't know anything about these), or you could even convert the videos to another format.