sync issues with MPEG 4 files

I just got through an edit where I received interviews shot from various sources. One of them were files (720x1280) that were MPEG 4. The edit went fine although I've noticed sync issues with those files. Not often, but enough to be annoying. Is there anything I can do to fix them? Should I have run them through a conversion of some kind before bringing them into fcpx? thx

It's hard to tell without knowing the complete specifics of the media, but you may very well need to convert before importing.

Right-click them, select Transcode, make Optimized media, as Tom says.

So if I already have the clips edited into the timeline and then I optimize the source media (I realize I can't optimize from the timeline itself, unless I'm mistaken), will I need to reinsert the optimized media into the timeline or will it automatically replace the unoptimized media with the optimized media? I'm, not sure if I'm making this clear.

I just imported a clip that is not QT, no proxie, no optimized.

Right-click, Transcode Media, created Proxie.
Changed from Optimized/Original to Proxie.
Works just fine.
Changed back to original, works just fine.

Right-click, TRanscode Media, created Optimized.
As soon as transcode was done, right-click, Reveal In Finder.
It showed me the Optimized QT ProRes 422.
Works just fine.

Quit FCP X, deleted Optimized QT ProRes 422.
Launched FCP X.
Right-click, Reveal In Finder, shows the original non-QT file.
Works just fine.

So yes, it all works just fine.

I'm optimizing my files as we speak. This looks like a good option. Thanks for taking the trouble to do all this.