Thanks to all that respond!

I have an 04 honda civic hybrid. I recently made a trip to the ozarks where there are many long downhill runs. My issue is that several times on a long downhill, the car would start to pulse (slow forward backward rocking). It would feel like a slow *jerking* of the car. I would not have my foot on the accelerator OR brake. The car was just maxed out on mpg and charge lights about 3/4. If I applied the accelerator, the pulsing would stop. This leads me to believe its a regenerative braking issue. If this is the only time it will ever do this, I am fine with it. But I am worried its a symptom of something going bad. Any thoughts?

For a little added info, my IMA battery was going bad and I swapped it out for a used one with only 70k (my car has 130k) about a year ago. I have not had IMA problems since the swap out. I also have no idea if this problem existed before because I never had long hills to drive on. Could be coincidence.
Thanks!
David