I need help !!!!

this is the problem :

SOMETIMES when sitting at idle (550 rpms) all my lights go dim and my amp gauge goes down almost to the red area and other times it stays up at almost thirteen with the lights glowing brightly. it doesn't matter which accessories are on or off and it can change from stop light to stop light without changing any of the operating accessories. I have already replaced the alternator and the battery, i have reconfigured the ground cable to go to the frame though a bolt and then to the block and made sure my battery connections are clean as well as my frame and block connections. I know you can't really trust the dash gauge but you can see the lights dim out when the gauge drops, so I know there is a problem. PLEASE HELP !!!

 

86,

Check the ground connection near the battery. There is a ground stud with a few connections. Remove the nut, clean the stud and connectiosn, apply some anti-seize compound, and reassemble the connection.

You may also have a problem at the starter positive power node. Disconnect the battery and do the same at the starter positive cable that you did at the ground connection.

Make sure the alternator power stud connection is clean and tight, but that isn't likely to cause dimming and voltage drop to the extent you describe.