  We study the lazy Markov chain on $\mathbf{F}_p$ defined as $X_{n+1}=X_n$
with probability $1/2$ and $X_{n+1}=f(X_n) \cdot \varepsilon_{n+1}$, where
$\varepsilon_n$ are random variables distributed uniformly on $\{ \gamma^{},
\gamma^{-1}\}$, $\gamma$ is a primitive root and $f(x) = \frac{x}{x-1}$ or
$f(x)=\mathrm{ind} (x)$. Then we show that the mixing time of $X_n$ is
$\exp(O(\log p / \log \log p))$. Also, we obtain an application to an
additive--combinatorial question concerning a certain Sidon--type family of
sets.
