fuel is good hi grade stuff from an F.B.O
Is it 100 LL or auto fuel? Do you run any auto fuel?
Do you have any mil-spec black fuel lines in the aircraft? Those fuel lines were supplied with many kits years ago before they knew there was a concern. Still finding them in aircraft flying today. They are known for swelling on the inside causing fuel flow restriction with no apparent changes visible from the outside. Might be a reason you can run awhile at higher rpms then it drops off. Just one place to start your search for the cause. Good luck.

Congrats. on a successful landing.