This is a commom problem with 1 wire alternators. The field needs 12 volts to activate the field windings to start the charging process. Low engine speed on start up is not enough to start the charging process. reving the engine actually will induce a voltage into the field windings to get thing going. I had a similar problem with my 1962 Ford galaxie with the 406 CID engine. I decided to ad a wire. I connected a 12 gauge wire from the second small terminal on the stater solenoid to the field connection on the alternator. This terminal is used only when cranking originally to provide 12 volts to the ignition coil on start up only. A second triger only terminal is found on most gm solenoids also. You can check with a test ligh. Have someone crank the engine. If the light lights only when crankig than this is the terminal. solves the problem. Yup, it is a second wire, but is only used to start the charging process. Who cares. Good Luck!
Paul Johnson
www.CustomClassicsAndPerformance.com