Potassium was my guess too. Whatever the problem it is manifesting itself as deficiencies but is surely due to nutes locking out. You are right to flush. Do like HappyBudder says and Flush...Drop your nutes down(400ppm could be slightly low) and then bring them back up. Get your nutes to about 900-1000 ppm and let it ride there wait a week and see if any defiencies show up. If they do, diagnose them and treat the problem with the proper supplements.
I do not think with tap water at 150 you should use Cal-Mag. I would try leaving that out until you see a problem forming. Nothing I have seen looks like Mag. I recently went through a build up and lock out situation and learned to flush with ph'ed water every other resi change.

Good luck -Jay