  We present results from timing observations of 27 pulsars made at the
Pushchino Observatory over 33.5 yr between 1978 July and 2012 February. We also
analyze archival Jet Propulsion Laboratory data of 10 pulsars to extend
individual data span to 43.5 yr. We detected a new phenomenon in the timing
behavior of two pulsars, B0823+26 and B1929+10, that demonstrates a rapid
change of pulsar rotation parameters such that the sign of the second
derivative $\ddot\nu$ is reversed. An analysis of the $\ddot\nu$ changes showed
that this process can be considered as a modulation process in $\ddot\nu$. We
showed that the process of rapidly changing of pulsar rotation parameters
represents a new type of rotational irregularity that, together with three
other types of rotational irregularities (discrete glitches, slow glitches and
quasi-periodic oscillations), forms a large-scale structure of timing noise.
These effects are all the cause of the deviation of the timing behavior of most
ordinary pulsars from a simple $\nu,\,\dot\nu$ spin-down model. We found that
all four types of observed rotational irregularities have evolving nature.
Irregularities in pulsar rotation rate pass through three evolutional stages
that show that a certain type of rotational irregularity can occur only at a
certain stage of pulsar rotation evolution. The age boundaries between
different evolutionary stages are indistinct and diffusive. This fact is
because different pulsars having similar properties evolve along different
paths. The evolutionary scenario of the occurrence of rotational irregularities
explains well many of the observed properties of pulsar rotation.
