It might be wise to look into a roller cam conversion at this point. Before my time but, the boat yard I worked at had issues with BBC flat tappet cams going flat. I know next to nothing about BB Chev, but it seems like they have some oiling issues. Add in suspect cam cores and renegade lifters with todays aggressive lobe profiles and stiff springs. If a roller cam isn't in your future, PLEASE talk to someone about getting a cam that's been nitrided or whatever proprietary process that Comp is using (parkerizing?). Yeah, the core may still be too soft, but its better than nothing. What are your valvetrain specs?
Sorry to see you're having problems like this