Thanks, Bob and Rick, for the suggestions. I think 3dZcutup may solve the problem.
I followed the procedures outlined in HowTo #5 pretty closely, including eliminating voxels outside the brain using Automask. Since slice 0 is still brain, I don't think voxels outside the brain would be contributing to this problem the way they would at more superior edges where brain borders skull. Please let me know if I'm misunderstanding something.
I'm now reasonably confident that this is motion and/or cardiac artifact. However, I wouldn't mind getting a second opinion. Let me know if someone would have time to look at it if I were to upload a scaled and unscaled run.
Thanks again.
Phil