A small shaft of grade 304 stainless is threaded through a collar and is failing a few threads into the collar. The application is the shaft has a solinoid pressing down on it about once a second. They are failing after about 100 service hours. How can I extend the life of these shafts?

The grade must be a non-magnetic stainless steel. I also tried reducing the number of threads, but the new fail point moved up the shaft back into the threaded area.

Any ideas how to extend the life? Thanks.