First, DONT USE a test light! Use a volt meter. thats the only way to get a true reading. Could be a dying battery. Went through the same thing. Also Check and make sure you have a good ground to the block from the battery the engine to the body and the body to thew frame. that should solve any problems. The altenator can be taken to any auto store and checked for FREE
PEP Boys, Auto Zone, Advance Auto Hope It RUNS
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66 Mustang Black W/ Black
289 Hi-Po 65'heads, MSD
Lunati High lift Cam
750CFM Carb Heddman Headers, Hurst Shifter