My power company delivers a perfect 120 at 60 Hz. I'm not worried about them, they're big enough to be sued so they can afford to do it right. Not everyone is on the national grid.
If your Generator is outputting 68hz then it has serious problems. We run quite a few diesel powered generators for center pivot irrigation and the most they ever get out sync is 61.5hz. At that point they get pulled and serviced.
The power at our mill and grain leg have never seen anything beyond 60.2hz and I'll bet that .1hz of that reading is instrumentation error.
It's a 8kW portable, the engine speed controls the frequency. It is a simple throttle adjustment to set it back to 60.