  We present the second XMM-Newton observation (85 ks) of the narrow-line
Seyfert 1 galaxy (NLS1) I Zw 1 and describe its mean spectral and timing
characteristics. On average, I Zw 1 is ~35 per cent dimmer in 2005 than in the
shorter (20 ks) 2002 observation. Between the two epochs the intrinsic
absorption column density diminished, but there were also subtle changes in the
continuum shape. Considering the blurred ionised reflection model, the
long-term changes can be associated with a varying contribution of the power
law component relative to the total spectrum. Examination of normalised light
curves indicates that the high-energy variations are quite structured and that
there are delays, but only in some parts of the light curve. Interestingly, a
hard X-ray lag first appears during the most-distinct structure in the mean
light curve, a flux dip ~25 ks into the observation. The previously discovered
broad, ionised Fe Ka line shows significant variations over the course of the
2005 observation. The amplitude of the variations is 25-45 per cent and they
are unlikely due to changes in the Fe ka-producing region, but perhaps arise
from orbital motion around the black hole or obscuration in the broad iron
line-emitting region. The 2002 data are re-examined for variability of the Fe
Ka line at that epoch. There is evidence of energy and flux variations that are
associated with a hard X-ray flare that occurred during that observation.
