it does sound as if the rectifier isn't working well.
you can replace rectifier cheaply enough from an ebay part.
however you may just have too much load on the alternator, mine for instance on my xr2 couldn't maintain charge on a 14.4v regulator, it was only producing 13.6-13.8v at best.
so i got a 15v regulator and voltage went up to a constant 14.4v, sometimes hits 14.5v, which is acceptable and charges much better, but i think i have too many electrics for a 90 amp alternator, still need to blip throttle on start up to get it running though, properly lazy, maybe the pulley is a bit big.
i'm thinking a 180amp vw one or fitting a clutch mechanism for the pulley for extra free power and less strain on crank nose during gear changing.