Coughing attacks

Question
I have a 2 year old Jack Russell/ Miniature Schnauzer mix who has had episodes of coughing, wheezing, and gagging for over a year now. I took her to a veterinarian twice when this first started happening but the vet could not find anything wrong with her. We've tried administering antibiotics in case it was kennel cough and Atarax to see if it was allergy related, but neither seemed to help her. She will breathe normally for most of the day but then at times, often when she gets excited, she just has a coughing attack. They don't last long but it seems she can't breathe well during these episodes. She doesn't seem to have trouble inhaling but wheezes and gags on the exhaling. I've tried to research this online and have seen people post similar problems searching for answers themselves. Some have concluded that their dog has asthma. Could this be the problem? Since she's already been examined by a vet, is there much point in persuing more veterinary visits? Please let me know if you have any ideas on what this might be. She is otherwise a very active, healthy dog with a good appetite and happy disposition.  

Answer
has your vet done xrays or a trachea wash yet?   those may be warrented.

OR
is this the kind of cough/sneeze that sounds like they are sucking air in?  thats called reverse sneezing and can be very WEIRD to see happen.

hope this helps some.