Warp threads breaking

I am having an interesting problem with my current project. I am weaving with 60/2 silk warp sett at 96 epi and 140/2 weft at 60 epi, on an AVL 24-shaft Workshop Dobby Loom. Initially, I had lots of problems with breaking threads. I was breaking a thread about every inch (or less!). Then I tried a few things, like opening the shed a little bit wider, sanding and waxing the (end-feed) shuttle until it was smooth as glass, and tightening up the beater so there was less play in it. I broke maybe five threads in the entire next yard of weaving. Now the threads are breaking again. In both cases, the broken threads were almost always on shafts 22-24, suggesting there is a problem there. The broken threads are distributed evenly across the warp and I don't feel any too-tight areas when I run my hand across the warp, so I don't think it's an issue with over-tight warp threads. I've made the same adjustments I made earlier and am still having trouble. So I'm at a loss to explain. What I don't understand is why the warp threads are breaking frequently at some points in the warp but not in others, and why the breakage is almost always in the last four shafts. Thoughts? I have 17 more yards to go on this warp, so any suggestions would be GREATLY appreciated!