Hard to say without looking at the project. However, there is a workaround: Set your comp to full-quality everything (full resolution, motion blur and frame blending enabled, etc), then press command/control with the Numeric 0 key to RAM preview -- once the RAM preview is complete it will prompt you for a place to save the file. Then it will write an uncompressed (Animation codec) movie directly from the RAM.
Of course, this is a temp fix, but sometimes for smaller projects it can get you through.
Good luck!
Ben Unguren
Motion Graphics & Editing
http://www.mostlydocumentary.com