If it makes you feel any better (probably not), my tank is is 6 years old and the same herbie setup since day one, and I have to adjust mine from time to time. Maybe not every week, but periodically. In some cases I do know what causes the variability, and in some cases I don't. Here are the causes that I know for certain changes the flowrate and overflow water level:
1. gummed up overflow weir. Left too long and it gets gunk or algae in it which restricts the flow. Algae detaches from it on its own and it will GUSH. Sometimes feedings get trapped in the gummed up overflow weir (algae trap).
2. Something gummed up in the gate valve. See #1 above
3. Using a filter sock. It does strange things to the flow and fluid dynamics.
4. Dynamic changes in skimmer water level. My skimmer is on its own set of mystery parameters where by it will occasionally change water level which in turn affects the entire flow and fluid dynamics of the entire tank.