Suddenly, just as this summer is beginning, my two three-year-old cats
have occasionally been "coughing".  

It starts with one "cough", then the cat quickly crouches so that its
body is sort of close to and parallel with the floor, and then the
coughs continue.  It's a similar position to the position cats adopt
when they are about to throw something up.  However, they are not
throwing anything up.  It also seems to be something to do with the
windpipe, as if there's some obstruction.  Their collars are not tight -
you can get two fingers under them.

Is this a case of hairballs?  Or what?

If it's hairballs, what do I do?  

So far they've always recovered after a dozen coughs or so, but it is
happening once every couple of days, and clearly they're in a state of
discomfort while it occurs, so I think if there's something I can do to
stop it, then I should.

Eddy.