I have done everything i can think of changed altornaters changed batteries( bought a optima)changed coil and plug wires, look for grounds and found some but i still loose voltage some how. it will be fine when i starte the motor up when the car is coldit will read bout 13.3 to 13.4 at idle and weill go don to bout 12.6 to 12.9 and evan farther down when i get higher up in the rpm range i have know clew what to do

 

I'd suggest a new fuel pump and pump harness from racetronix especially if you have an older car. Also a new ignition switch can really help alot also.

 

Belt slipping,loose connection,bad diode in alternator.