Are your data items of wildly different magnitudes? Year and year cubed are less of a
strain on the optimizer if rescaled to (year-1980) and (year-1980)^3 (assuming 1980 is the
mean year). As a bonus the coeficients are easier to understand. Scale the data so that all
variables fall into similar ranges.
There are several -maximize_options- that can be useful. See
for the Stata documentation, but here are my sugestions: