Alternator dipping

[Nixon1]

Lately I've noticed at a low idle my alternator's output, according to the gauge, seems to drop. When in gear, after I've been sitting at a light or something for a fair amount of time, the output on the gauge drops by about 3/4 of a bar, then back up again.. It's not spiking, it's dropping, since it holds the higher output longer than the dips last. Out of gear, like in park, it also drops, but more drastically, but also at a less frequently and unpredictable rate. There is no change in the car's idle in correlation with this, although the idle is not as smooth as I would like it, fluctuating by about 50 rpms continuously and missing occasionally. The alt output never dips below the normal area, but usually it is on the lower end of normal. Is this normal behavior or is something wrong? Thanks!