My 8 year old Honda 3000i did this not too long ago.. I've always kept gas in it and I do exercise it from time to time, but it was surging in eco mode with no load. Even with a little load, it still surged.
I turned off the eco and let it run full speed and put about 1500 watt load on it and ran it for about 4 hours (I plugged it into my house gen switch and flipped the switches).
It used about 2 gallons of gas during that time! I've ran it for 36 hours before (on eco) on the house and it only used 5 gallons of gas..
Anyway, after doing that, the next time I exercised it, it idled smooth as ever...
I think it just needed to run 'hard', kinda like "blowing out the carbon" on your vehicles engine every now and then... ;)
Dunno if that was it or not, but that's my story!
Mitch