If your car won't start because the solenoid won't energize, you may have a wire gauge problem. If you have an automatic with a neutral safety switch in the console, change the wires attached to the switch to 12 gauge. The existing wire is too small and there isn't enough juice getting through to the starter solenoid when things are hot. Heavy gauge battery cables will also get more juice to the starter itself.

Great information, thanks. Are you refering to the early Chevelles when you mention changing the wire gage? In 64 the solenoid "S" wire was 14 gage. By 67 it was 12 gage. Don't remember which year it changed, but Chevy must have upped the size for the same reason you found.