If your Sundowner 50 is a model originally intended for a .50-size two-stroke engine, then the .95 FX would be considered grossly large. That would encourage air being introduced into the fuel by the engine's vibrations. This air will lean the mixture and cause the engine to die.

Put the engine on a very solid test fixture and see how it runs. If it runs "all day", then there are no problems with the engine, iteself, but in the engine/airframe/fuel system combination.