Motor runs great wide open but dies at idle/idle speed?

It's a 15 hp Mercury that's about 4 years old and my neighbor is the original owner. He has never had issues until this summer. The motor doesn't want to sustain running at idle speed. When put into gear at the slowest speed possible, it was the same way. It would eventually start hesitating and sputter off after a maximum of 15 seconds. However, opening up the throttle and going any speed after slow, it ran like a champ.

I don't know a whole lot about engines, but could this be a bad/dirty spark plug? The gas was from last summer, so the first thing he did was drain it and put fresh fuel in the tank which wasn't the whole problem at least. The motor was used twice last year - to put it on the lift in May, then to put on the trailer in September. So I assume that these issues are just because of inactivity. Any ideas of what it could be, and how to solve it?