5D2 Max Rez for Raw Video recording WITH Sound?

[66dellwood]

I have only recently been trying to record decent sound with raw video on my 5D2.  I have good in-camera results with a BeachTek preamp and various microphones with h.264 files.

When recording at 1856x1044/23.9 fps I seem to be loosing audio sync.   When I down rez to 720p/ 23.9, sync is fine.

Does 1856x1044 with sound overtax the 5d2?

Curious if anyone has experience with various resolutions and sound recording.