Here is a link at Pro Plan's website to get a $3 coupon. I buy mine at PetsMart and Pet Supplies Plus (who is a little cheaper).
http://www.proplan.com/products/requestCoupon.html

Glad Gizmo continues to do better. I would definitely contact Nutro just in case there is something to it. My cats have eaten Nutro for years with no problems but just like with people food I'm sure you can sometimes get a bad batch.

Hope the Pro Plan works for you if you decide to try it. It cost about the same, maybe a little more than Nutro.