  We present thermopower $S$ and resistance $R$ measurements on GaAs-based
mesoscopic two-dimensional electron gases (2DEGs) as functions of the electron
density $n_s$. At high $n_s$ we observe good agreement between the measured $S$
and $S_{\rm{MOTT}}$, the Mott prediction for a non-interacting metal. As $n_s$
is lowered, we observe a crossover from Mott-like behaviour to that where $S$
shows strong oscillations and even sign changes. Remarkably, there are
absolutely no features in $R$ corresponding to those in $S$. In fact, $R$ is
devoid of even any universal conductance fluctuations. A statistical analysis
of the thermopower oscillations from two devices of dissimilar dimensions
suggest a universal nature of the oscillations. We critically examine whether
they can be mesoscopic fluctuations of the kind described by Lesovik and
Khmelnitskii in Sov. Phys. JETP. \textbf{67}, 957 (1988).
