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I'm a little worried that you're dealing with bloat. Bloat is where the stomach twists in the abdomen. It occurs commonly in older large breed dogs. We don't know why it happens, but we see dry heaving, gagging, retching, abdominal distension and labored breathing.
Bloat is fatal pretty quickly if not treated surgically. I would want to get an x-ray to make sure this is not happening. If it is, then surgery is curative and we tack the stomach to prevent recurrence. If it's just some gas in the stomach, then I would use Simethicone (Gas X) to see if that helps the signs.
Here is a nice link on bloat in dogs:
I hope this helps, let me know if you have any other questions.
Sorry about that wait- I had to go see a case.
Oh, I didn't know this was a recurrent thing. In that case, I would guess it's more of an indigestion issue. It could be acid reflux. It could be some gas distension and bloating (but not twisting). Ideally I would get abdominal x-rays while the signs are occurring. That will tell you more about what's going on in there.
My guess is that some Gas-X or Pepcid (or any antacid) should help to just treat symptomatically when it occurs.