Hey Lee,
Looking at the picture I'm wondering if that first black border may be where it starts to get wavy. It could be that your border is just a little too long. One way to fix it would be to take the border off, remeasure the length through the middle and use that measurement to redo the border.
Or, you could try blocking it. To do that you need to get it damp and then pin it flat to a flat surface and let dry. Sometimes that will solve a problem if it's not too pronounced. This is the first thing I would try.
I've also had a piece that flattened itself out due to shrinkage of the batting after I washed it.
and, I had one that became wavy after quilting-- that one just needed some additional quilting to get it to behave. I don't think that is the problem with your piece though since it is not densely quilted. But if you did some piano key quilting in the border that might draw it up enough to get it to lay flat without having to take the border off and recut it. you'd still need to redo your binding, though.
Rob