but it isnt doing it all the time!

alternator is shafted fela as said above, no point should it be below 13.6, ideally 14 min.

Car turned on with everything on (lights/heaters/wipers/screen heat)..12.03v..........no good
Car turned on..11.97v.....no good
Car held at 3000rpm with everything on..12.88v ............no good