Epsom salts can be dissolved in water at a rate of 1 T per gallon and then drenched around the plant to see if it responds to the magnesium. That won't harm the sago. Your plant's symptoms also can point to soggy soil conditions or temporary drought stress. Try to maintain moderate soil moisture, try also the epsom salts treatment and it should recover and resume normal growth although the yellow leaves may not regreen much. But there should not be new leaves that start to yellow.
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