I had the EXACT SAME PROBLEM, AND SCENARIO ten or so years ago.
I used to service work and had this, in a trailer park in Concord CA. Turned out to be a U/G direct burial feeder had a nick, and the AL conductor rotted over time.
To save on digging, put a tracer on the neutral and hope you get a dead end to the signal from both sides near the same spot. If you do thats were the Christy box goes. Becacuse it is intermitant they is no real guarantee it will would but well worth the shot!
Another method is to TDR the run, or use a pulse generator on the run. If you dont have any of the varied expensive equipment to find it, there are companies that specialize in it that you can sub it out too. Or you can rent it...
"Well - I oughta....." -Jackie Gleason