What size fuel line are you running? It sounds like high-speed fuel starvation. I had this same problem until I switched from medium to large fuel lines and replumbed the fuel tank to accept larger brass tubing (don't forget to drill out the clunk too).

I personally never ran a fuel pump, but opted instead for a main tank (on the CofG) and a header tank under the cowl. It started easy, ran great and kept the same needle settings from the start of a flight to the end. I also always ran it with a fuel and air filter (and after-ran after every flying session), so the piston/sleeve is still as smooth as day one (some carbon build up, of course), without any scoring.

Daren