Can you tell if the engine is an iron piston in steel sleeve, or an aluminum piston in a nickel-on-brass sleeve? (Different fuel needs according to the metals...)

What fuel are you using? (Too much or too little castor in the blend may cause problems...) An ABN 35FP should be happy on a 10%Nitro/11%synthetic/11%castor fuel, and should probably run in a low-2-cycle mode throughout the flight. The iron in steel version probably would like 22% to 25% MOSTLY castor fuel best.

Is the engine very hot after a flight?

I've heard of occasional inaccuracies in commercial fuel tanks - split or badly located fuel pickup tubes for example - that can cause problems. A crack in the uniflow vent tube inside the tank may uncover about halfway through the flight, and change the effective needle setting - usually a tad richer... A tank with the pickup tube or either vent tube not well soldered to the shell can suffer the effects of the tube 'whipping' around a small amount inside the tank shell, foaming up the fuel and affecting the setting -usually seeming leaner.

Do you see any bubbles in the fuel line while the engine is running? The could indicate a fuel foaming condition which could go worse in flight. Some cushioning under the tank might improve vibration insulation.

Of course, you could try different plugs, props, settings... Something that basic may just do it.

Luck!