For some reason zinc and magnesium products have this effect on a small portion of the population. There are various theories as to why but they are all unsubstantiated. One thing you may try is taking the dose about an hour before dinner. This may help out. If not then move the dose more towards the middle of the day.
As a last ditch effort you might try taking the product until you go to bowel tolerance and then cut the dose back. For instance try taking 5 tablets one night and then 6 tablets the next night. See if you can saturate your system and then cut the dose back to normal. Loose stool will be the sign that you are saturated with magnesium. If the loose stool takes a while to subside just take a calcium tablet, a glass of milk or other high calium food or product and it will stop the loose stool. You might be really mineral deficient for some reason.
Let me know how it goes.
-Matt