Donna, have you used fresh yeast? Now, I've never used sourdough (though am a bit interested, I will admit), but I find fresh yeast much more "forgiving" than the dry stuff.
This is even though I'm generally happy with dry yeast. I just like the feel of the dough more when I use fresh yeast. The gas bubbles seem to be finer somehow.
I'm sure some people will be surprised at how I keep my fresh yeast, but it works very well for me: I buy 500 g (about 17 oz) blocks of fresh yeast, remove the paper wrapping (because it sticks when the yeast is frozen) and double wrap it tightly in plastic bags. I then freeze it. When I want to use the yeast I remove it from the freezer and hack off the amount I need (I weigh it), put the block back into the freezer, add the warm water to the still frozen yeast and proceed as normal.
Rose L. Beranbaum says to defrost the frozen fresh yeast, but I have never done that, and find I don't need to.
Anyway, just one more idea. (Maybe one of these days I'll try a sourdough culture, but I think that if I did in my kitchen, I'd just get commercial yeast strain.

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