Artifacts in filtered video

I have imported SD anamorphic footage from several cameras from a brightly lit stage performance. One of the cameras was a consumer camcorder on a locked-down long shot that has brightness and contrast problems (small bright people on large dark background made the people too bright, too much contrast). So I am trying to use video filters to adjust those shots to better match the other cameras.

Using the Quicktime Brightness and Contrast filter plus the Quicktime Sharpen filter, I found settings which look pretty good. And on some of the shots, those render and play fine. But on other similar shots, when I render with the same settings, I get an unstable image, with flashes of varying brightness/contrast and color.

Is there a way to eliminate these artifacts? Or are there other filters that produce the desired change without such artifacts?