After the recent outage I ran my LL Hearth and Coal Trol with my Champion gen 3500/4000W. Started the gen up with the choke and left it at half choke as it would stall if I tried to turn the choke all the way down. Went about my business of running extension cords around the house. I have a Kill-A-watt meter ($20) plugged in to an extension cord and went to check it. Voltage was 120V. Great. Frequency was 75Hz. 75Hz? I didn't know what to do. Went out to check the gen and realized the choke was still partially on. Turned the choke off. Magically the frequency went to 60Hz thereafter.
Another thing I noticed was the blower on the back was stuttering - Running very slow and speeding up and down. It was not noisy and ran very quiet. Disconnected it from the coal troll and plugged it into 120V direct. Ran great then. I guess coal troll does not like my champion.
My summary would be to keep an eye on the voltage AND frequency. Watch the fans for noise AND speed.
Sure wasn't magic
Sounds like something is wrong with your generator! Needing to use the choke very long suggests a fuel feed problem. (crud or water in carburetor) and frequency changes sound like a engine governor problem. That could easily cause speed variations, thus frequency changes. The AVR (automatic voltage regulator) can't compensate for engine speed variations which determine frequency. Overly lean engines tend to run over speed unless they stall.
Actually, the frequency change you observed might not be any actual change in frequency! Kill-A-watt units are very simplistic and just count voltage zero crossings. Any brush-slip ring intermittent will cause extra zero voltage drop outs (not true crossings but that device can't tell)
A significant problem with generators is they often go unused for some time. Lack of use messes them up. Modern "gasoline" with ethanol loves to soak up moisture and eventually precipitate it. Water usually can't get through the carburetor fixed jets. Long gone are needle adjustable jets one could "open up" to pass crud.
Try using double the amount of Stabil or even better, marine Stable. Marine Stabil has improved water handling characteristics.
You may be able to free up the governor simply by cycling the load on the generator from near full capacity to no load.
A good guideline with generators is to run them every 3-4 months. No load to significant load. It keeps the fuel system clean and governor from sticking. This also keeps the brushes from sticking and the slip rings from building up corrosion or crud that will pit them under load.