  First, we review the current status of the detection of strong `external'
variability in the CLASS gravitational B1600+434, focusing on the 1998 VLA
8.5-GHz and 1998/9 WSRT multi-frequency observations. We show that this data
can best be explained in terms of radio-microlensing. We then proceed to show
some preliminary results from our new multi-frequency VLA monitoring program,
in particular the detection of a strong feature (~30%) in the light curve of
the lensed image which passes predominantly through the dark-matter halo of the
lens galaxy. We tentatively interpret this event, which lasted for several
weeks, as a radio-microlensing caustic crossing, i.e. the superluminal motion
of a micro-arcsec-scale jet-component in the lensed source over a single
caustic in the magnification pattern, that has been created by massive compact
objects along the line-of-sight to the lensed image.
