What kind of false bottom are you using? With that much grain, I think you have a gap in your false bottom. I find that backing off the mill gap reduces the quantity of grain that sneaks through. I also runoff through a mesh bag so any bits get held up there, but I measure my grain bits in teaspoons, not cups. I had a round Igloo brand cooler that I used as a mash tun. The bottom would warp a little allowing lots of bits through. I now mash in a keggle with a domed round false bottom.