Not saying that this IS your problem, but I'm facing a similar issue and I'm pretty confident it was due to a bad batch of salt... Took me a long time to discover it and unfortunatly things got pretty bad. Mag and Calc were WAY high, Alk was low. I should have known better when the salt had an unusual texture and it took more than my usual 2.5 cups / 5gal to reach the desired SG.
The moral of my story is measure your freshly mixed water and make sure it isn't going into the tank already messed up.
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