Q: Final cut pro

What format are you exporting and how are you playing the file back?

x

What is the format of the footage you are editing? What camera does it come from? Did you convert it to ProRes, or is it something like H.264?

Quicktime is a framework that supports a number of compression/decompression strategies (codecs). What specific codec are you working with?

H.264 is one of many but is not supported in FCP for editing.  As noted by Shane, you should be editing in ProRes (usually LT is fine for most consumer camera material) then doing any conversion to a delivery format after exporting. You can use Compressor to convert.

x