Another thought...
Fiberglass itself is a bit abrasive. If you sanded the string groove and left it bare thinking you might have to sand some more, it might be rough enough that it is "sanding" your string. I'd sand it again with the finest paper you have, 600 grit or finer, and then spray a coat or two of clear over it and let it dry well and try it again.
Also, look very closely if you have multiple tip overlays. Possibly, if one of the layers was just barely cut through when the groove was filed and if the glue joint there is bad, maybe there is an edge that is moving when under tension at full draw and sort of opens up and pinches the string? A drop of thin super glue allowed to wick into the cracks and then clamped should help for that.
Lastly, are you making your own strings? Any chance you are doing something wrong like getting more tension in one bundle than the other? That would put the entire load on one half of the bundle an might cause it to start giving out pretty quickly. I'd think it would be pretty obvious if it was uneven but I'm just throwing out ideas in the off chance it might help.