Carlos,

I know the WM T-34 is used for racing in some circles. But having a thick, high lift symmetrical wing, it is built more for great maneuverability and for slower flight. It lands, under power, as slowly as most trainers.

My T-34 (with an OS.46FX) flies much better with a Bolly 11.5x6 and even with a Master Airscrew 11x6, it flew better than with a 10x7.

I suggest you change to an APC 11x6. It will load your engine more, but vertical performance will be improved visibly.

Your muffler equipped .50 (the engine is not rated by the manufacturer with the muffler...) is probably exceeding its RPM of maximal power, with the 10x7 prop you now have. Maybe this is the reason for the surge.

Is your fuel tank completely insulated from airframe vibrations, by a 1/4" layer of latex foam? Maybe vibration levels increase as the prop unloads in flight, causing the fuel to foam, or the clunk to take flight.

About your needle; if and only if you see bubbles in the fuel line between your needle and your carburettor, that are absent in the fuel line between the tank and the needle, is there a problem to be rectified. Check at all RPM levels.

The best fix, if this condition occurs, is to replace the O-ring that surrounds the needle.
Don't bother to find an OEM part. Any neoprene, or silicone O-ring, of the correct size will do.

If there is a leak in a fuel nipple, within the remote needle unit, exercise your warranty and replace the remote needle unit.