I have had that same problem on some of my own, (I didn't want to spend any money, raising 2 kids) and rather than buy a higher output alternator, I would turn the idle up just a little.
You will want to make sure that the belt is tight, (duh) if you have not already.
Then, sometimes the alternator may test good, (either on the bench or in the vehicle) but the actual load that is put on it when the car demands it might be causing the amps to go low and then eventually the volts will follow.
If the idle seems a little low, you might want to just tweek it up a bit. Belt tight, (not glazzed too too bad) and the correct alternator for the application, should be good to go.
If not, then you might want to see if there is a higher amperage for that application.
Oh...also a good fully charged battery....