  We report measurements of the longitudinal ($^{139}T_1^{-1}$) and transverse
($^{139}T_2^{-1}$) decay rates of the magnetization of $^{139}$La nuclei
performed in a high quality single crystal of
La$_{1.65}$Eu$_{0.2}$Sr$_{0.15}$CuO$_{4}$. We observe a dramatic slowing of the
Cu 3d spins manifested as a sharp increase of both $^{139}T_1^{-1}$ and
$^{139}T_2^{-1}$ below 30 K. We find that in this temperature range the
fluctuations involve a unique time scale $\tau $ which diverges as $(T-T_{\rm
A})^{-1.9}$ with $T_{\rm A}\thickapprox 5$ K. This behavior is distinct from
the continuous freezing observed in underdoped La$_{1-x}$Sr$_x$CuO$_4$ which
involves a distribution of energy barriers. By contrast, in
La$_{1.65}$Eu$_{0.2}$Sr$_{0.15}$CuO$_{4}$, the freezing below 30K is intrinsic
to its magnetic ground state and the observed power law supports the existence
of a glass forming "charge stripe liquid".
