Some dogs do get bronchitis or even pneumonia but that doesn't really sound like the problem or I don't think he would have episodes of feeling great....running around....eating happily....etc..... Anything inside the chest would usually require an x-ray and sounds like you and your vet have been on top of it so I'd think he did one already.

Speaking of......when you do see him have a problem...check carefully for the respiratory effort I was talking about. Is he having trouble getting air IN (inhalation?) or pushing the air back OUT (exhalation) Effort on inhalation usually means some problem in the upper respiratory tract....nose, trachea etc.... while effort on EXHALATION will lead one much more to the thought of a lung/chest issue.

With your (cheap) stethoscope make a habit of listening to his lungs, throat etc....then listen again when he's having problems. Where is that sound the loudest....his chest or his throat? Pinpointing that part will go a HUGE way in solving the puzzle.

Let me say that troubles in EITHER area can cause severe breathing difficulties but knowing your pets vital signs really helps your vet pinpoint what's going on......we hope.

PS...as far as pain control.....when he is having pain issues check his gums....sometimes (not always) SOMETIMES you will see their gums actually get more pink than usual....sort of a coral/reddish color due to increased blood pressure. Usually you will see the animals panting heavily if you are seeing this particular thing but not always. This would not be something I would consider normal either....just fyi.