  Aims. We have searched for temporal variations of narrow absorption lines in
high resolution quasar spectra. A sample of 5 distant sources have been
assembled, for which 2 spectra - VLT/UVES or Keck/HIRES - taken several years
apart are available. Methods. We first investigate under which conditions
variations in absorption line profiles can be detected reliably from high
resolution spectra, and discuss the implications of changes in terms of
small-scale structure within the intervening gas or intrinsic origin. The
targets selected allow us to investigate the time behavior of a broad variety
of absorption line systems, sampling diverse environments: the vicinity of
active nuclei, galaxy halos, molecular-rich galaxy disks associated with damped
Lya systems, as well as neutral gas within our own Galaxy. Results. Absorption
lines from MgII, FeII or proxy species with lines of lower opacity tracing the
same kind of gas appear to be remarkably stable (1 sigma upper limits as low as
10 % for some components on scales in the range 10 - 100 au), even for systems
at z_abs ~ z_e. Marginal variations are observed for MgII lines toward PKS
1229-021 at z_abs = 0.83032; however, we detect no systems displaying changes
as large as those reported in low resolution SDSS spectra. In neutral or
diffuse molecular media, clear changes are seen for Galactic NaI lines toward
PKS 1229-02 (decrease of N by a factor of four for one of the five components
over 9.7 yr), corresponding to structure at a scale of about 35 au, in good
agreement with known properties of the Galactic interstellar medium. Tentative
variations are detected for H2 J=3 lines toward FBQS J2340-0053 at z_abs
=2.05454 (~35% change in column density), suggesting the existence of structure
at the 10 au-scale for this warm gas. A marginal change is also seen in CI from
another velocity component (~70% variation in N(CI)).
