Quoted from Fish:
but once in a while, when I hit a drop target, the playfield lights dim and then the playfield light fuse blows.
So you'll be looking for a short in the lighting circuit. Common causes are bulb socket tabs (the part with the cloth-covered wire attached) bending into the socket, or twisting over and touching another piece of metal. Since it happens with a drop target hit, maybe start by looking carefully around the drop target area bulb sockets for anyplace where metal could be touching one of them.
These types of shorts can be very difficult to spot though, so it helps to go very methodically through each socket and connection to verify there's no stray metal touching.
Other techniques get more involved, and a lot more intrusive (by isolating the circuit segments).
A clue can sometimes be found by determining which light is being lit by the event prior to the fuse blowing (since the path to that bulb is being activated, and the short occurring). So when you hit a particular drop target (or after a series), if it's the same light that then comes on, check that one carefully.