EF1000IS is giving problems

I've had my Yamaha generator for about 3 years and it has decided to run badly. It is surging constantly or maybe the term (govener hunting) would better describe my problem.
 We use it while camping to power my wifes c-pap machine and have had no problems until a week ago. It's much worse when running a very light load when the engine is at it's lowest speed.
 I have changed the engine oil annually and kept the air filter clean and lightly oiled as per the manual. I have also cleaned the exhaust screen, but nothing has helped. A new spark plug gapped .026 also hasn't solved the problem.
 Do any of you have a suggestion as to what might be causing this, Thanks Glenn