I have a similar experience. I have a 2 year old fuel valve, recently balanced the carbs and been on 3 long road trips without any problem. Until today. It´s 30+ degrees C, very hot and I found it sputtering and stalling 3 times on the way home. I switched the fuel valve to PRIME and then it ran fine for a while before dying on the side of the road. I filled it with cool fuel but it didn´t help. I also noticed the high heat coming from the engine. I have a feeling it´s something to do with vaporizing fuel from the heat. Spring and fall I have no problems at all. I haven´t taken it to the shop yet. I´ll wait for a cooler day and see if it makes a difference.