Jeff, the area outside of the image area gets the most exposure, hence contributing to the results you're seeing. Basic exposure time should be judged according to that area, the 'base + fog' area of the negative. I mention this because there are several ways in which people test for basic exposure time. That is, unless the area outside of your image is receiving direct UV with no blank negative laying over it. This will vary depending on whether you're using analog negs or digital ones...

If the problem exists outside the image area only, and you don't care about seeing brush marks in the borders, you can mask the perimeter with rubylith or red lithographers tape to eliminate them. Your clearing process must be very good (or use a dbl layer of tape or ruby) in order to avoid a grayness in that area.

Another thing to try, that I swear by, is to sandwich the coated paper/negative combo between the contact frame glass and a piece of mylar in order to prevent loss of humidity through the back of the paper. This will also help dMax...