Apparently there is a durability problem with some of the older Zipp hubs. I rebuilt this pair of 404 Firecrest wheels with Chris King R45 hubs and Sapim CX-Ray spokes back in January and I have done three more sets just like it since then.
If you have a set of Firecrests with hub issues feel free to give me a call to discuss options. Zipp makes one of the best carbon rims on the market. Don’t let them sit around collecting dust, they have a lot more life left in them