keylight and crashing

I am using a Mac Pro dual quad with 14 Gigs of ram. In AfterEffects CS3 I am trying to chromakey SD component video recorded on Beta, captured on an Avid, and exported to me, as Meridian 1_1 compression Quicktimes. I am using Keylight.

I can get a good looking key during preview, and when I switch an unaffected top layer on and off I don't see any change in how the video looks (except for spill suppression.) Yet when I look at the final render output, the chroma keyed footage appears soft, and there is color saturation loss, and a lot of noise visible. I have tried (Quicktime) Avid Meridian 3_1 and 1-1 compression output, and finally rendered the comp out as a pict sequence, and then rendered that pict sequence back to an Avid quicktime: in each final render the keyed footage looks soft and noisy while the background plate (from a pre-rendered uncompressed movie I made in AE) looks clean and sharp.

Another problem (hence the pict sequence renders) is that After Effects keeps crashing during the render when I am using Keylight. The source quicktimes are quite long: the shortest is about 1:30, and the longest is almost 15:00 (minutes.) At least with a pict sequence, I can pick up the render from the last frame. Is there an optimum file size for the source quicktimes? Am I trying to render files that are simply too big for AE and my computer to manage?

I have looked through the forums but haven't found any problem exactly matching mine, and was hoping to see if anyone else has had such problems, and what their solution might have been.

Thanks