Excess Magnesium can be very drying. You may need to balance the Mg with some calcium. Try to take some calcium with vitamin D, at least 1000mg per day for a day or 2 and see if it helps with the diarrhea. Try not to supplement any Mg for a few days just to help speed the balance. When the diarrhea stops you may want to change to a 2:1 ratio of cal:mag. You may also want to supplement with 200-400IU of vitamin E and fish/flax oil to help with the psoriasis. However, be sure to use the E if you decide to use any EFA oils or they can make your skin condition worse.