Is $(a,g^{ab})$ computationally indistinguishable from $(a, g^c)$?
Their statistical distance is less than $\: (q\hspace{-0.04 in}-$$\phi$$(q))\hspace{.02 in}/q\:$, $\:$ since

$\:$ if $a$ is relatively prime to $q$ then $(\hspace{.02 in}g^a)^b$ and $g^c$ are

$\:$ each uniformly distributed and independent of $a$
$\;\;\;\;$ and
$\:$ even if $a$ isn't relatively prime to $q$, $(\hspace{.02 in}g^a)^b$ and $g^c$ each

$\:$ have a positive probability of being the identity element

.
Therefore, if $q$ has no small factors then they are in fact statistically indistinguishable.