Fuel Starvation / Mechanical Fuel Pump

Mechanical fuel pump is fine. You have a restricted line or it's vapor locking.

Could you show pictures of your fuel line routing from carb to frame rail?

Did this problem just start? Sounds like you had this problem before and changed the fuel pump. If so, how long before that, and what changes since start of problem?

Could even be float setting.

Have you ever seen junk and debris in your fuel filter?