Check your brush gear or the sensor wire in your alternator. both caused the symptom you described in my pintara. all so it could be the regulator, prob best just to replace the regulator and the brush gear in one go if it's a hybrid regulator like mine.
Just make SURE you disconnect the battery before you mess with your alternator. leaving the battery on when messing with the regulator is what killed my sensing wire! (100 % dickhead factor on that one!).
When driving at night (away from city lights) do your lights flicker or slowly go bright then dark/dim?
All so your battery could be on the way out, or as suggested by u13 bluebird[nic-34u] it could be a loose connection between your alternator and your battery, or just a loose/corroded battery connection.
I probably put those items in reverse, check the battery/alternator connections first.
All so check if something is discharging it when the engine is off.
Good luck!