  Using new techniques based on the polarimetric fluctuation spectrum, the
fluctuation behaviour of the polarization of individual pulses is examined in
three pulsars that show drifting subpulses, allowing various aspects of the
fluctuations to be quantified for the first time. Of the three pulsars studied,
only PSR B0809+74 shows behaviour completely consistent with the superposition
of orthogonal polarization modes (OPMs), and this only at 328 MHz and in
superposition with an apparently randomly polarized component. The observed
periodic pattern is decomposed into the sum of two orthogonally polarized,
out-of-phase drift patterns, one of which shows a dramatic jump in subpulse
phase near the leading edge of the pulse window. For PSR B0320+39 and PSR
B0818$-$13, considerable periodic fluctuations away from OPM orientations are
seen, a condition that also occurs in the trailing half of the pulse in PSR
B0809+74 at 1380 MHz. In some cases the deviation is so strong that the
periodic locus of the polarization vector in the Poincar\'{e} sphere is almost
circular, in contrast to the strictly colinear states of superposed OPMs.
Several possibilities are discussed for the physical origin of these patterns.
The similarity between the subpulse patterns in one of the OPMs of PSR B0809+74
at 328 MHz to that of the total intensity signal at 1380 MHz supports a picture
of superposed, out of phase drift patterns. To explain the full range of
behaviour seen in the three pulsars, it must be possible to produce at least
three arbitrarily polarized superposed patterns. While the data do not suggest
a particular approach for the empirical decomposition of patterns into
non-orthogonally polarized components, the specific, quantitative nature of the
results should provide strong constraints for theoretically driven modelling.
