Thanks so much for the suggestion. I've now reset the Preferences and cleaned the cache files. I am getting the same results. The Auto Colour dumps out half-way through the video clip. It is very strange as I have longer clips right after it that have full Auto Colour correction throughout without any problems. I've tried using other effects like Proc Amp and they work, but not to the same degree and quality as Auto Colour, so I'd like to use AC if possible.
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Perhaps you can provide a summary of which parameters in Auto Color you are keyframing. Is it possible that you have altered Blend with Original to 100% when your clip loses its auto color? View the timeline in the Effect Control panel and see if any of the values under Auto Color change when you play the timeline.
(Just some thoughts.)
Thanks for the ideas. I appreciate you taking the time to respond. I didn't have anything set in the timeline. I just dropped autocolor onto the segment and liked what I saw without any other adjustments. I eventually cut the segment at the point at which it kept changing and used the Color Balance control on both segments. For some reason, Auto Color would just not work on the second half of the segment.