I would assume it's due to the lower, warm idle as opposed to when the idle is raised when cold. By 15k you could certainly help your cause by removing the dist. cap and cleaning it, as well as the rotor if anything is built up on it. If that car has an EGR valve, maybe that's got something to do with it, but I don't know much on that topic.

Since you say it misses now and then, first thing I'd do is look under the cap. If it looks alright and cleaning it out doesn't help I'd take a look at the plugs to see their condition.

You say it does it for 15 sec. Where are the rpms and do they increase after those 15sec?