  In order to study precession and interstellar magnetic field variations, we
measured the polarized position angle of 81 pulsars at several-month intervals
for four years. We show that the uncertainties in a single-epoch measurement of
position angle is usually dominated by random pulse-to-pulse jitter of the
polarized subpulses. Even with these uncertainties, we find that the position
angle variations in 19 pulsars are significantly better fitted (at the 3
{\sigma} level) by a sinusoid than by a constant. Such variations could be
caused by precession, which would then indicate periods of ~ (200 - 1300) d and
amplitudes of ~(1 - 12) degrees. We narrow this collection to four pulsars that
show the most convincing evidence of sinusoidal variation in position angle.
Also, in a handful of pulsars, single discrepant position angle measurements
are observed which may result from the line of sight passing across a discrete
ionized, magnetized structure. We calculate the standard deviation of position
angle measurements from the mean for each pulsar, and relate these to limits on
precession and interstellar magnetic field variations.
