Too much sour in sourdough

Too often, my sourdough has a bitter, vinegar-like aftertaste that can linger on the tongue an hour or more. 

I don't think I'm overproofing but I'm puzzled as to whjat causes this. Most of the time I do final proofing overnight in the fridge. I usually use King Arthur bread flour and a locally m,illed whole wheat flour. 

Does it matter what my starter is? I've thought about buying some of the S.F.-style yeast on line, but I've always thought that was a bit of a gimmick in that a starter will eventially take on whatever years is floatiung aroubnd locally. 

Any help is appreciated.