More "funny" problems with export to .mov

Hello

I have recently been experiencing problems and strange behavior, when exporting my projects to H264 mov files with the "Export with compressor setting".

One problem is, that some video clips are getting slightly out of sync. Now this is easy to tell, as I have been filming myself playing drums. I'm not sure that I could tell from a speech clip. I am not having these issues inside FCPX, were playback is perfect.

Another problem I have encountered is that the volume of some clips are quite a bit lower after export than when I playback in FCPX.

Have any of you guys had similar difficulties?

Thanks!

What are your hardware specs?
How exactly are you making the H.264?
How long is the clip being exported?
What are your peak and average levels on the meter in FCP X?