If your video is 29.97fps and was originally shot on film, it's probably been telecined. Telecine is a process that changes 23.976fps (frames per second) progressive video to 29.97 frames per second interlaced (59.94 fields per second). Normally that means you can perform an IVTC (Inverse Telecine) operation on it to recover the original progressive frames. Not only will this give you back the original progressive content, it will also reduce the filesize by discarding 1 out of every 5 frames.
Note: Since the IVTC process creates 23.976fps, depending on your final format, you may need to add pulldown information after encoding. There are some free utilities available for this, and some DVD authoring software will do it for you when you load 23.976fps video that hasn't already had pulldown flags added.
The Decomb Plugin
IVTCing your video requires 2 filters, both of which are in the Decomb plugin. Decomb also comes with excellent HTML documentation (which is also in the AVSEdit Help File), including a tutorial which has step by step instructions for performing IVTC and a Reference Manual with hints for using each of the included filters. If the steps in this guide give you poor results, the tutorial may help you sort out your problem. If you have a hybrid source (mixed film and video content) you should go straight to Decomb's documentation. If your source is pure film you can (usually) use the steps in this guide.
Note: If your source is a D2V file and you used Force Film when you created the project, you've already performed IVTC on it. If you want to use AviSynth to IVTC you should create a new DVD2AVI project with Field Operation set to None.
The first step is to recreate the original progressive frames. Before you can do that, you need to determine whether your video is TFF (Top Field First) or BFF (Bottom Field First). This can be done using Core filters from the Interlaced category. Add AssumeTFF and SeparateFields to your script and preview it. Navigate through the preview until you find a section where something moves either left to right or right to left across the screen and step through several frames, one frame at a time. If the object moves backwards in every other frame, the field order is BFF. If the movement looks correct it's TFF.
Delete the AssumeTFF and SeparateFields lines and add the Telecide filter from the Interlaced category. Set the order parameter to 0 if your video is BFF or 1 if it's TFF. You must set the order because there is no default value. Preview your video again, find a section with motion, and step through it one frame at a time. You should see a repeating pattern of 3 unique frames followed by 2 identical frames. If you don't see this pattern, or you only see it some of the time, refer to the Decomb tutorial instead of this guide.
Now that you've recovered the original film frames, you can use the Decimate filter from the Interlaced category to get rid of the duplicate frames. You don't need to set any parameters, as Decimate drops 1 in 5 frames by default. Make sure you preview your video to see if there are any problems. Once again, if you end up with video that doesn't look right, refer to the Decomb tutorial for more help.