can be a short within a socket itself. look at the sockets for easy tip offs like solder splashes... remove the bulbs.

Then start breaking the string up into smaller segments if you can't find the culprit. It's probably in a socket tho if you haven't found it tracing the wires.

Although you did a playfield swap, I've had a case where the fuse holder itself was the culprit. The GI works for a while, then the fuse gets so hot from the slight resistance in the holder, that it blows the fuse.

is one of the socket leads accidentally touching something it shouldn't be?

Also be sure to VERY closely inspect the pop bumper socket leads under the playfield. They can very easily touch the pop bumper brackets and short out.
If you get desperate you can pull all the bulbs in that circuit and see if it pops, then add them one by one.