I recently changed spark-plug wires on my SB 66. That's all, just plug wires. I drove it a few times and one day it wouldn't turn over. Battery good, no crank on the starter motor. All I did was new wires, so I pulled off the shielding over the distributor to see if I yanked something loose. I checked everything, and all looked good, got in the car and it started up. Scary. A few weeks passed and I decided to reinstall the shielding and very carefully installed being sure not to pinch anything. All went well for 2 more weeks then again, no cranking. I'm thinking it has to be the ignition switch, as there's nothing on the firewall to cause this (wrong!). I again removed the shield, wiggled the wires for kicks, and it starts up. That's it, I said, I am going to buzz every wire on the firewall and find the broken wire. Maybe I should have mentioned the car began life as a Powerglide. The neautral safety switch goes from the shifter up through the firewall into plug-ins (purple wires) along the firewall. When I removed the PG, I merely jumpered the connections where the switch was and left all of the wiring in its original location. One of the plugs had been partially removed for who knows how long, and was just getting by. I put a little pressure and it seated. I went ahead and opened up the connections, cleaned them with contact cleaner and put all back. Yes, it started, but did I find the problem? I will let you know in a couple of weeks, if the schedule is consistent. Let this warning go out to PG conversions.