As already said by frenchelectrician, sectionalise and megger all cables concerned and check the insulation resistance between cables to lamps. Hopefully you don't have breach joints in the SL circuits. These usually get damp because of poor sealing of the outer sleeving. The increased voltage will cause them to break down faster and hence intermittand faults once in a while.
From control point lightcell contactor, fit section fuses on each outgoing SL circuit.
Also check the overall loading per string of lights.
Fit fuses in each lamp acces cover if not already there.
The product of rotation, excitation and flux produces electricty.
I would account for all the runs of wires in case the origional circuit picked up an remote outlet or some other light like a lighted sign or something. Being that the voltage was boosted, if anything was left on circuit, it is safe to say it's toast. Eliminate the easy things first before megging the wires or adding fuses. Do you recall the weather conditions a few days before the outage(s)?