  We analyse dispersion measure (DM) variations in six years of radio
observations of more than 160 young pulsars, all gamma ray candidates for the
Fermi gamma ray telescope mostly located close to the Galactic plane. DMs were
fit across 256 MHz of bandwidth for observations centred at 1.4 GHz and across
three frequencies -- 0.7 GHz, 1.4 GHz, and 3.1 GHz -- where multifrequency
observations were available. Changes in dispersion measure, dDM/dt, were
calculated using a weighted linear fit across all epochs of available DMs. DM
variations were detected at a 3\sigma level in 11 pulsars, four of which were
above 5\sigma: PSRs J0835-4510, J0908-4913, J1824-1945, and J1833-0827. We find
that after 28 years of gradual decline, the DM of PSR J0835-4510 is now
increasing. The magnitude of variations in three of the four (PSRs J0835-4510,
J0908-4913, and J1833-0827) are above what models would predict for an ISM
dominated by Kolmogorov turbulence. We attribute this excess as likely due to
the pulsar's local environment - the supernova remnants near PSRs J0835-4510
and J1833-0827 and the pulsar wind nebula around PSR J0908-4913. Upper limits
were determined for all pulsars without detectable values of dDM/dt, most
limits were found to lie above the levels of variations predicted by ISM
theory. We find our results to be consistent with scattering estimates from the
NE2001 model along these lines of sight.
