Mark Mandell - Gone Round In New Jersey
Barry, you are becoming sensitized to the walnut. It is a progressive condition; the more exposure, the worse is the reaction. You can actually put yourself in danger of anaphylactic shock if you keep on. You can use all of the measures you mentioned and then add moonsuits so you don't drag the dust with you on your clothes when you leave the shop, but once sensitized you stay that way.
Your option is to find another wood to build with.
I've got a large pile of walnut stacked in my basement. So far, I've not been effected, but I've taken the advice of several woodworkers and always used a good mask, constant dust collection, shop cleaning, and vacuuming my clothes when done.