Sounds like the overall tension in the wheel is too low, causing cyclic unloading and "worrying" wear on the NDS spokes ( which are most likely to go slack since they start out under the least tension ). Triple butted spokes ( like Alpine ones from DT ) will tolerate this a little better than most, but tension is probably the root cause. Let a wheel builder look at it if you don't build wheels yourself; your rim might be strong enough to support just pulling the whole wheel up in tension, or you might need a stronger rim back there.
The fact that it was ok for years bears out the tension hypothesis, so you will want to respoke at least the non drive side before pulling the wheel tighter: all the spokes on that side probably have the wear that has killed the few you have lost already.
Last edited by MrEss; 04-30-12 at 10:46 AM.