  In this paper, we describe a novel experiment for the accurate estimation of
pulsar dispersion measures using the Giant Metre-wave Radio Telescope. This
experiment was carried out for a sample of twelve pulsars, over a period of
more than one year (January 2001 to May 2002) with observations about once
every fortnight. At each epoch, the pulsar DMs were obtained from simultaneous
dual frequency observations, without requiring any absolute timing information.
The DM estimates were obtained from both the single pulse data streams and from
the average profiles. The accuracy of the DM estimates at each epoch is ~ 1
part in 10^4 or better, making the data set useful for many different kinds of
studies. The time series of DM shows significant variations on time scales of
weeks to months for most of the pulsars. A comparison of the mean DM values
from these data show significant deviations from catalog values (as well as
from other estimates in literature) for some of the pulsars, with PSR B1642-03
showing the most notable changes. From our analysis results it appears that
constancy of pulsar DMs (at the level of 1 in 10^3 or better) can not be taken
for granted. For PSR B2217+47, we see evidence for a large-scale DM gradient
over a one year period, which is modeled as being due to a blob of enhanced
electron density sampled by the line of sight. For some pulsars, including
pulsars with fairly simple profiles like PSR B1642-03, we find evidence for
small changes in DM values for different frequency pairs of measurement, a
result that needs to be investigated in detail. Another interesting result is
that we find significant differences in DM values obtained from average
profiles and single pulse data.
