Portable generator has low power output?

I have a Rural King 3500e (3500w) portable generator. It runs fine. I was using it at my work during a power outage and noticed that it was having issues keeping a small freezer cold. I tried it on a microwave and it the microwave was not receiving enough power to run properly. At times it would run fine, then would almost drop down to no power. I know nothing about generators other than how to turn them on and plug them in. Where would be a good place to start diagnosing the issue?