This is really weird, I am having the same problem, except once I get to 5500 my L28 starts to sputter and go back down in RPMs.
Swapped FPR, Fuel Pump, new filters, new ignition coil, still the same problem.
I can't put my finger on if it is fuel or spark related. I should be getting a wideband soon and then I can figure out if it is fuel related or not.
Since I am EFI and you are carb'd and we have the same problems I wonder if this would point to a spark issue?