As they age the smell declines, so stinky is good. Have you tried: tuna with the kibble or salmon, EVO reduced fat, Barking at the Moon, Innova senior, parmesan cheese, boiled or rotissere beef or chicken (if boiled, add a little garlic powder)
I've had dogs do this when their stomach is queasy. Are you giving any stomach meds? pepcid, tagamet, zantac, carafate, prilosec?