  We report the flux measurement of the Vela like pulsar B1800-21 at the low
radio frequency regime over multiple epochs spanning several years. The
spectrum shows a turnover around the GHz frequency range and represents a
typical example of gigahertz-peaked spectrum (GPS) pulsar. Our observations
revealed that the pulsar spectrum show a significant evolution during the
observing period with the low frequency part of the spectrum becoming steeper,
with a higher turnover frequency, for a period of several years before
reverting back to the initial shape during the latest measurements. The
spectral change over times spanning several years requires dense structures,
with free electron densities around 1000--20000 cm$^{-3}$ and physical
dimensions ~220 AU, in the interstellar medium (ISM) traversing across the
pulsar line of sight. We look into the possible sites of such structures in the
ISM and likely mechanisms particularly the thermal free-free absorption as
possible explanations for the change.
