Are you planting them in the same place every year. Sometimes the soil is to blame. I have heard that you should rotate where you plant tomatoes every year or two.
I agree w/Bet or even don't plant any for a couple of years. Or put them in containers only in a different area. Some folks have had luck doing that. You might want to check with your extension service too to see if you can do anything in the off season for control.
"The soil is the source of life, creativity, culture and real independence." David Ben-Gurion
