So let's see it sounds like the platter when removed lays flat. The bearing when you check it turns true. So the problem is probably what teo_audio outlined, the platter got bent, probably somewhere near the center.
What you need then is some way of bending it back straight. Easiest would be if you can lay it down upside down on something solid and flat and then tap it out flat from the underside. Hold something like a dowel or block of wood to the high spot and hit it with a hammer. Don't hammer directly, you'll make dents or maybe even crack the platter.