My elderly EE has periodically been having this issue - she will be pecking around and suddenly sneeze/cough and take a few breaths with her mouth open, then go on her merry way. She doesn’t appear distressed when it happens, and she’s done it before and then stopped for a few months before starting back up again. Eating, drinking, and holding her own with the rest of my girls. She’s virtually impossible to catch so I haven’t been able to give her a good once over but she appears to be in good health overall for a 7-8 year old chicken.