I have a 2004 4Runner and I had the same problem. I noticed it when I first purchased the vehicle. Not only did the seat pop around, but the ride height of the seat would gradually reduce after each corner I took. It continued to get worse over the last two years. And my 3/36 warranty was getting ready to expire so I took my 4Runner to the dealer (Power Toyota in Tempe, AZ). They told me that they had seen a few of these. It was under warranty and they replaced the entire seat - they said that was the only fix for it. They also said that retail on that seat is about $2500 (because of built in airbags). Anyhow, that's what one dealer did. Go get your new seat!