Reno,
Another source of spikes is an occasional lightning storm in the area. You mentioned parking lot lights. I have seen damage in parking lot fixtures and also several hundred feet inside buildings when poles attract the big sparks. That is why some engineers require a ground rod in the concrete base of metal parking lot poles.
Another possibility could be faulty underground 12.5kv utility wiring coupling to parking lot wiring.
Have you checked the impedance of the ground and neutral for the house panel? Ideal makes a fairly inexpensive tester ($200) that can be used to test branch circuit and feeder ground impedance.
On the subject of switching those outside ballasts, in my experience a contactor or heavy duty lighting relay seems to work more reliably than a photocell only. If the photocell is not conducting the full sine wave, there could be harmonics created by the photocell. If the photocell has a relay, the contacts are usually pretty small. Some photocells use solid state relays which could be source of your problem if they are acting like a dimmer-only conducting for a percentage of the sine wave.
And a final suggestion, what is the voltage drop on the circuits at the actual load?
