Posted a similar thread before but Ive narrowed it down and have more info now...

I'm having a fuel delivery/pressure/volume issue. 1994 454 TBI. This is in a jet boat.

I swapped fuel pumps, hardwired the pump straight to the battery, installed an Aeromotive regulator, had all new fuel lines made, high flow fuel filter...nothing. The motor cuts out like its losing fuel around the mid range of the rpm band and then near WOT and at WOT is picks back up and again and runs perfect with zero miss. The digital AF gauge reads lean at the same exact time the motor cuts out and the fuel pressure gauge right before the TBI on the inlet side drops pressure. I'm pretty convinced that there is a fuel delivery problem (as opposed to an ignition/injector problem) but I don't know how or why it's only in the middle areas of the RPM band and at WOT it screams with no issue. Easing into the throttle doesn't make any difference.

The ID of the fuel lines are 3/8". The Motor is a 454 that has a mild cam/stock iron heads/overbored TBI/.030 over cylinders with 10:1 compression SRP forged pistons. I only run 91 octane. The fuel pump is the $300 962 Carter/Indmar Marine fuel pump. For approx 375HP this should be plenty of fuel pump.

Throw out ideas please!