CalmReason, Why wouldn't you get lip-sync?
This is an issue when aligning a separate sound-track (eg from a Zoom/PalmTrack recorder).
However, you can experience sync issues in "Preview" mode, since the PC needs to be high-spec to keep up with the video data, let alone the audio. You need to render and burn a DVD to check sync, using a stand-alone DVD player and TV . . . not your PC.
Mixing camera and external is unlikely to work, giving an "echo" effect.
Can you explain what you are doing, gear spec and how this Lip-Sync error manifests itself?
Without that, we're left guessing?