I have been working on a forecast model in Excel extrapolating from a small (150 data points) monthly time series. I've converted into a year/year percentage change series to get it stationary, deseasonalized it with monthly dummies, and accounted for serial-correlation (it has a two period error correlation).
However, after graphing the residuals like so...
...it's very clear that there is non-seasonal, semi-regular periodicity that still needs to be removed. What is the recommended approach for modeling this kind of error (hopefully one that doesn't require R)?
Edit: The PACF for the first two lags were (1) -0.481, and (2) -0.2786. PACF values for lags greater than were not significant.