Are you using hydrogen peroxide? Not to repeat myself, but I SWEAR by a capful of 3% hydrogen peroxide in 20 ounces of distilled water for seed starting. I used to constantly fight damping off and since I started using it I've went 100% germ rate and survival rate on all mature seeds. Not just GPS (26/26), but stubborn seeds of various perennials as well. It could be something as simple as a pathogen on the outside of the seed that attacks the taproot as soon as it sprouts. For your own sanity, please give it a try.