pdxtex,

It will not always be the loosest spoke that breaks next. It will be the one that has accumulated the most residual stresses at the elbow. These stresses are heightened when one or two spoke break and other spokes are instantaniously required to take the extra load until the wheel is re-tensioned. Think of the wheel as a balanced assembly in which all spokes have an equally distributed workload. When one ore more are not doing thier share of the work, some of the other spokes in the assembly are required to take up the extra load for a time. These spokes will experience a bit of extra stress untill relieved by retensioning. These stresses add up over time and are exposed as material failures when the elbow (usually) fails.

The continuous spoke breakage is a symptom that tells me IMHO that your wheel is sadly in need of a rebuild. You are essentially replacing all of them one a time but paying the labor once is less costly and you have a dependable wheel. Buying new wheels should include a good tensioning by a competent wheel man as well because most of these "sale" wheels are machine built and are not necessarily uniform in tension. You essentially are staring the same cycle all over again but just won't likely see results very soon. The key to longevity is a properly tensioned wheel meaning the correct amount of balanced tension. Machines rarely get this right but the wheels are very cost effective. So do what many do. Buy the wheels and get them "touched up" My last set are approaching 5000 trouble free miles. Your other option is to rebuild with new spokes. If you like the rim and it is still serviceable, this should be fine. My personal choice is double butted spokes as the "shaft" of the spoke takes some of the shock away from the elbows further reducing stresses that are introduced. This contributes to the longevity of the assembly.