" I also had been taking relatively high magnesium...sometimes 800 mg or a bit more."
-I can tell you that 800 mg of magnesium can cause all of your stomach symptoms alone. It took me 3-4 years to be able to absorb my magnesium supplements, started this before iodine. I am now at 3-500 mg; this was out of the question just a couple of years ago. Diarrhea, gurgling, all kinds of stomach problems.
Type and size matters.
The time-released supplements I stay far away from; they typically contain like mentioned, dicalcium phosphate, magnesium stearate and other nasties.
Yes, quit everything for 5 days, as V suggest. I needed to several times. Then on again, slower this time:)