Nobody said this yet, but are you positive your problem isn't fuel foaming in the tank?

If possible, I'd repeat the test on a stand, or use an external fuel tank (which you can see that isn't connected to the airplane) and see if it still does this.

All my Saitos have run great from the box without any tweaking to get them RTF. 7 of 8 have needed to have their low speeds adjusted leaner ~ 2 turns after "break in", however the latest one is the best of them all and didn't need any adjustments besides the usual warm/cold weather needling.