1999 ford windstar won't start

1999 ford windstar no spark. I changed out cam sensor, ran fine for a week. then no spark. changed cam again, ran fine about a week. no spark again, let set a week, started right up and ran, so I was told the crank sensor might be knocking out the cam sensor, so I replaced crank sensor. van ran well for 3 months, now I have no spark again. what could possibly be making these sensors keep going out?