Since this also happens outside of Final Cut (Motion exports in DV),
I suspect a flawed version of Quicktime or perhaps more ominously,
a hardware (RAM?) issue. The problem has persisted since a clean
re-install of OSX and all other applications.
Thanks for the Smart Anamorphic filter suggestion. This is what
I was looking for. The distortion ramp towards the edges is really
steep however (steeper than what my Panasonic monitor does) and there is no
way to adjust this. Any other filters like this out there?