  Combining measurements taken using the Wilkinson Microwave Anisotropy Probe
(WMAP) from 2001 to 2008 with measurements taken using Planck from 2009 to
2010, we investigate the long-term flux density variability of extragalactic
radio sources selected from the Planck Early Release Compact Source Catalogue.
The single-year, single-frequency WMAP maps are used to estimate
yearly-averaged flux densities of the sources in the four WMAP bands: Ka (33
GHz), Q (41 GHz), V (61 GHz), and W (94 GHz). We identify 82, 67, 32, and 15
sources respectively as variable at greater than 99% confidence level in these
four bands. The amplitudes of variation are comparable between bands, and are
not correlated with either the flux densities or the spectral indices of the
sources. The number counts of WMAP Ka-band sources are stable from year to year
despite the fluctuation caused by individual source variability. Most of our
sources show strong correlation in variability between bands. Almost all the
sources that show variability are blazars. We have attempted to fit two simple,
four-parameter models to the time-series of 32 sources showing correlated
variability at multiple frequencies - a long-term flaring model and a
rotating-jet model. We find that 19 sources (60%) can be fit with the simple
rotating-jet model, and ten of these also fit the simple long-term flaring
model. The remaining 13 sources (40%) show more complex variability behaviour
that is not consistent with either model. Extended radio galaxies in our sample
show no sign of variability, as expected, with the exception of Pictor A for
which we report evidence for a millimetre flare lasting between 2002 and 2010.
