How do I fix this? (Chroma Key Rendering Issues...)

I'm currently using chroma key to key out a green screen background, and the footage looks awesome in the preview, but the moment I render it, everything gets blocky and pixilated *just* around the edges of the subject (where the green screen foreground meets the background). The footage is 4:3 NTSC that I recorded on an XL2 at 24p. I'm trying to then place it on an anamorphic 16x9 sequence, crop the edges of the green screen footage, remove the rest with with chroma key, and make it one of several elements in my anamorphic 16x9 palette, but this pixilation issue is driving me nuts and I can't seem to figure out how to fix it.

I've removed everything from the sequence except a solid color background (from the standard video generator effect) and the pixilation is still present. I'm guessing there's something I'm not doing in the render settings or sequence settings or something maybe that I *needed* to do in the capture process to make this work but the technical options of FCP are admittedly not my strong suit.

Does anyone have any ideas as to what I need to do to make this work? Any ideas would be appreciated.

Thanks!

Could you post a sample screen?
But don't forget miniDV is not good format to work when doing Chroma Keying.