The amplitude of the two polarizations started to diverge an hour or so after the start of the experiment
The same affect is seen in both IFs and on 3C279 and 1830 - so its instrumental, rather than source related
at ~1250 UT there was a sudden degradation in the phases, followed by a rapid drop in amplitude of about 25%
The system is rephasing every 15 minutes at the start of a scan for both 3C279 and 1830-211