It’s going to be difficult and time consuming but youre going to need to do the diagnostics WHILE you’re having the problem. Drive it until the stalling starts to happen and try to keep the problem “alive”. Check anything and everything you can during the stumble. A few things come to mind, heat related electronics failures (coil, ballast, etc) or fuel pickup sock clogging, Both common. You tried three carter style (edlebrock) carbs, all of those have fuel boiling/percolating issues. If you can try a Holley style carb.
