Wavy borders can happen in the mitering , or another much more common way is when using cross grain , rather than lenght of fabric to make the border , it stretches when you are sewing . A few ways to solve is to starch , to help keep fabric from stretching( but do be careful when handling as to not over handle) . If you measure each side , and cut to that lenght , and pin the entire border so to be sure it is exact this can help. The stretch that occurs in cross grain can be almost unnoticable when working with small units in a block , that with the final block squaring makes the quilter sometimes unaware of what really caused the distortion.
It can be tough to make a border lay flat with cross grain ( becuase there is more stretch in cross grain). The problem seems to get more magnified with the wider the border gets. In an effort to "force it into flatness" it is tempting to steam the heck out of it and repeat ironings , this can often make it worse. Steam can be effective in reducing the stretch that has already occured , but needs to be combined with blocking to really be effective.
I had made a few quilts with multiple borders and those that were cut cross the width of fabric never seems to lay flat against the ones that where lenght of fabric. The lesson learned was to never mix the two in my borders. Batiks ( most) seem to have much less stretch on the cross grain, and I have the least amount of issues in wide borders with batiks using the cross grain.
You can take up the wave in the miter seam, but do pin first the excess at the seam to be sure this will be a satisfactory cure.
Sorry for you problem , DO NOT GIVE UP!