this was handled via email, but here was my answer:
You should have better luck if you grab the initial image in higher resolution, then downscale it with smoothing. so, try Scale = 2 or 3 or 4, then do this with the image:
var newimg = new Bitmap(newWidth, newHeight);
using (var g = Graphics.FromImage(newimg))
g.SmoothingMode = SmoothingMode.AntiAlias;
g.InterpolationMode = InterpolationMode.HighQualityBicubic;
g.CompositingQuality = CompositingQuality.HighQuality;
g.PixelOffsetMode = PixelOffsetMode.HighQuality;
//as high quality as we can get!
g.DrawImage(img, new Rectangle(0, 0, newWidth, newHeight));
newimg is the scaled down one...
img is the one you get from GoDiagram at scale = X