I think your main problem is that the profile curves are not closed. Meaning they actually have a start and an end point that are not the same. To see this, select one of the curves, and type the command 'crvend' and then 'crvstart'. This will create a point at the end, and start of the curve. Then zoom in real close to see that there is actually a gap/overlap there. Your curve at the edge of the front surface and the back surface is similar, except the ends overlap a little (about .008). That's why the sweep on the front surface 'looks' ok. It actually isn't.
I tried to fix it, but some of the control points overlap the line of symmetry. My best guess is that you mirrored one side to make the other, and then joined them? Looks like your curves overlapped the center line before you mirrored them, thus creating overlapping lines. Where is the curve you used to make the front surface?
There is a simple fix, but will require a little more work to smooth out the curves. You can turn on the EDIT points of the curves and select on the end, and have it 'snap' onto the other end. This would actually make the center point of the curve slightly off center though. I actually did this, and it worked (sort of). It may be best to start over and avoid this to begin with. If you need any help, feel free to email me (dab_design AT sutv DOT com) what ever files you have.