Silly question, what is the RPM range of the cam???

1. Fuel pressure, easily checked by installing a fuel pressure gauge and making a quick test run. If you're dropping below 6 psi you have to resolve the issue.
2. Try a run without the air cleaner, I got my backside kicked years ago by a big block that had to small of an air cleaner on it. Another similar situation was due to the air cleaner lid being too close to the top of the carb due to hood clearance.
3. Try un corking the motor. A restricted exhaust will drive you nuts. Think, mouse nest, collapsed baffle, heat riser stuck 1/2 shut etc.
4. Valve springs. I am not familiar with the 303 lunati but if it has steep ramps, springs, weak or not will cause the problem. IF that is the culprit it may also be prone to burping gas out of the carb due to the valves floating.(should be apparent with the hood and air cleaner off. One can make a series of test runs to establish the exact RPM that the problem occurs. Install .060 t.125 shims under the springs and repeat the test. IF the RPM moves up you have found the issue.
5. Along shot would be other valve train issues, IE: geometry, rocker arms/ interference, pushrod length etc.

keep us posted