JG,
You have two possibilities:
1 - It's in the light fixtures or panel
2 - It's in the runs
As you give no hint what we are actually talking about, this is all just wild assed guesses and it is the best answers you will get.
To find and repair a case 1, open things up starting at the panel and look for someplace a bare wire is touching something it should not. If you did not find it, then do the same at each light fixture.
To find a case 2, you am up THE creek bud. Want a hail-Mary try? You should now know where all the wires for those circuits run. Try disconnecting any one run and see if the problem goes away. If it does, you have an assemble issue. One of the runs got crimped or stapled where it should not have.
This is the sort of thing I fixed in cheap power cruisers before the depression. If they still have a boat, they can't afford me to fix it now, but I still get asked.
Good Luck
Matt