My '95 Wagon front seat back does this as well. More pronounced on the right-side than the left-side of the drivers seat, but it does do it.

What I find even more annoying is when the armrest is down, it shifts up and down. Drives me crazy in stop and go traffic. It doesn't do it when the seat back is more vertical or more reclined than normal.

I have been wondering if the gears in the hinge are worn. If so does this mean completely disassembling the whole seat to fix it?