Riva (along with nearly all other flv transcoding programs will fail when the video has a lot of still frames. Each identical frame will be merged into the space of one.

There is a program called Moyea Flash to Video Converter I found a while ago which uses a live capture of the FLV file. That means it doesn't have the same problems as all of the other converters. I do believe it costs money though, unfortunately. Possibly you could find a similar product.