I also had problems similar to this which I attributed to insufficient wort oxygenation. Since I started using oxygen (1-2 minutes through a .5 micron steel stone), this has not been an issue.
Further, I understand that you used a starter in some cases, so you might balk at the need for aeration / oxygen, thinking you had enough yeast to begin with. Take a moment to consider how you prepared your starter. Starters vary, of course. Did you shake it, stir it, let it sit? How much wort did you use in the starter? In other words, did the starter have sufficient yeast for your beer to fully attenuate? Starters can multiply yeast a little or a lot depending on method, wort and other variables, so consider stepping the wort up a bit, shaking more (or ideally stirring regularly such as with a stir plate) or otherwise improving your method.
Good luck - you'll figure it out! And the best part is, you get to figure it out while making more beer!