I see no harm in trying 50:1 and seeing if it produces an improvement. However IIRC you had it running perfectly at 25:1 for a while, so it sounds as if the fault is either marginal or intermittent.
The carburetor has different systems in operation at idle and at high load/speed. It apparently gets slightly lean in the transition between the two. If there is no air leak into the crankcase or intake port, the most likely source of the problem is the carburetor. Does it have progression holes, or just a single idle discharge port? This is the one or more tiny holes right at the throttle butterfly. If there are several holes, it may be that the ones furthest upstream toward the venturi, are damaged very slightly or obstructed. If there is only one, my guess is that you are asking too much of the carburetor. It is very common with the diaphragm carburetors on chainsaws and brushcutters to have to set the idle mixture a bit rich, to get the engine to respond smoothly at part throttle. You might try noting exactly where the idle mixture adjusting screw slot is pointed at present, then turning it anticlockwise a bit (toward richer) and seeing whether the mid-range running improves.
