  Bright single pulses of many radio pulsars show rapid intensity fluctuations
(called microstructure) when observed with time resolutions of tens of
microseconds. Here, we report an analysis of Arecibo 59.5 $\mu$sec-resolution
polarimetric observations of 11 P-band and 32 L-band pulsars with periods
ranging from 150 msec to 3.7 sec. These higher frequency observations forms the
most reliable basis for detailed microstructure studies. Close inspection of
individual pulses reveals that most pulses exhibit quasiperiodicities with a
well-defined periodicity timescale ($P_{\mu}$). While we find some pulses with
deeply modulating microstructure, most pulses show low-amplitude modulations on
top of broad smooth subpulses features, thereby making it difficult to infer
periodicities. We have developed a method for such low-amplitude fluctuations
wherein a smooth subpulse envelope is subtracted from each de-noised subpulse;
the fluctuating portion of each subpulse is then used to estimate $P_{\mu}$ via
autocorrelation analysis. We find that the microstructure timescale $P_{\mu}$
is common across all Stokes parameters of polarized pulsar signals. Moreover,
no clear signature of curvature radiation in vacuum in highly resolved
microstructures was found. Our analysis further shows strong correlation
between $P_\mu$ and the pulsar period $P$. We discuss implications of this
result in terms of a coherent radiation model wherein radio emission arises due
to formation and acceleration of electron-positron pairs in an inner vacuum gap
over magnetic polar cap, and a subpulse corresponds to a series of
non-stationary sparking discharges. We argue that in this model, $P_{\mu}$
reflects the temporal modulation of non-stationary plasma flow.
