From time to time (usually around 9pm to 12am) they will start cycling fan speed from very low to very high over the course of about 30-45 seconds
i was going to say daniel's point regarding the firmware is probably it, but then given that it often happens in a certain time span, it could be an external factor like voltage drop or something ?
regadless, the easiest way would be to try another firmware as a first step.