Probably an issue with the fuel delivery, likely some gunk or tiny debris in the carburetor. "Modern" ethanol fuels have a shelf life of 90 days or less. You could try running a fuel system cleaner like SeaFoam through the tank; this can (sometimes) be enough to clean the carburetor and restore a smooth-running engine. If that doesn't work, the carb will likely need to be disassembled and cleaned or just replaced (a new carb is only $20 or so). Not a difficult DIY job to swap out a carburetor on this model.
Does your HRX have an auto or manual choke? If manual, does keeping the choke ON smooth out the engine? This is a classic sign of too little fuel flow, often due to slime or gunk in the main jet / nozzle inside the carburetor.
