UK Car Repair: 2000 XK8 charging system, modern cars, charging system

Question
Over the course of the last year the charging system appears to be losing voltage.  When stopped at a streetlight with the ac running, lights and (obviously) brake lamps on, the radio starts cutting out, like is does not have enough current to run it.  At first if I just put it in neutral and let my foot off the brake the problem abated.  I replaced the battery, to no avail.  I just had the alternator replaced, now I get a warning lamp stating the battery is not charging.  Usually it goes away if I rev it up.  The nominal voltage at the battery never seems to dip below 13, with everything running, but the dash gauge routinely drops below the 13 mark at a stop.  Advice most appreciated.  Thanks,  Bill.

Answer
When the alternator was replaced was the output checked.  On modern cars the voltage rate is normally above 14 volts.  Is the drive belt correct, is it routed properly and is the alt pulley correct -maybe too large- or is there something else draining the system when it is running?