start with the simple things first. Check and clean the battery terminals, then check the starter connections. Also check the wiring for breaks and bare spots that could possibly short out to the engine or frame. If this doesn't do it, then onto investigating other problems like a bad solenoid or starter.
I’ll pose a question to the masses. Do the ’67-8 cars have an ignition lock out to prevent the car from being started in gear? If so that might be a source of a intermittent problem.
