ok, well it could be a number of things.
heaves is sort of like asthma. her airway gets inflammed, and she would have a hard time breathing. its accompanied by "heaving" flanks and a cough. Heaves does not typically present in a horse that is only 6 years old. For the person that wants to refute: that means that you have to feed your horse moldy dusty hay about everyday of its life.
theres also something i call runner's cough, that is when unfit horses work hard, without warming up. its like running your mile in gym, but never preparing for it first. your lungs ache, and you cough.
also, there is stable cough, when things get trapped in the horses airway, like sawdust, or hay dust, and the horse coughs to clear his airway to breathe properly.
one more thing, and that is get his teeth checked. sometimes if they are packing their cheeks, those little food parcels can dislodge from his teeth, and fly into his airway, causeing his airway to inflame and him to cough. (just like if you swallow something the wrong way).
roaring is very commonly seen in TB's not QH's. (unless its an appendix).
CoughEx works really well for stable cough, and heaves management. ask your vet the best way to go about things.
if you could provide a more accurate description of when it happens, and what it sounds like, that will help i think to get better answers.
PS _ Ron's a little sour... but he's right!
and btw! im not 12, im 13 and a half!!!!!! =P (i wish i was 17 again honestly)