The fact that your ambush squads mostly consist of lashers is somewhat unlikely, but not impossible. There's nothing you can really do about it, though.
The ambush squads aren't actually "appearing within minutes of one another," or rather, they are, but not the way you're thinking. When ambushes show up, all of the squads spawn at the map edges at the same time. The fact that they almost always get detected at longer intervals is a side effect of this and their transit time to your fort entrance, since each squad of ambushers is detected individually.
Finding a particular goblin to kill can be tricky. First, use your announcements screen to zoom to the dwarf that is getting interrupted, then see if you can't find the interrupting goblin manually. If you can, go into your (s)quads menu, select a squad with a, b, c, etc., set up for (k)ill orders, and then scroll over the offending goblin and hit enter. This will produce a kill order for that squad, meaning that all its members will activate and go hunt down that creature to kill it. Obviously, if you don't have any military squads, this won't work.
The other option to shutting that dwarf up is to make sure that no jobs are being taken where the goblins will be seen. This is most easily done by setting up a burrow and using alerts to restrict civilian dwarves from going outside the main fort region when a particular alert is set, meaning that until you declare it safe they won't go to do such jobs.