You should run it at least 2 hrs a month (continuously) at 1/2 to full load. Best guess is the carb is gummed up. The Seafoam may fix it. The carbs can't be rebuilt, and you will have to pull the gen-set to replace it.
Also, use a Kill-a-watt meter to check volts, and make sure the frequency is at 60Hz.
It may just be running too slow, so is stalling when it gets a little load. There is a screw adjustment on the carb linkage to speed up, or slow down the gen-set.
Make sure the altitude adjuster is set correctly.