I agree with Jerry. How thick are your cables coming off the battery? Bigger in diameter the better.

I had this exact issue, 0/0 cables, rebuilt and tuned generator and voltage regulator, and Optima battery, no more issues.

SWEET
Sergeant At Arms: Old Dominion Stovebolt Society
BUNS?!?!?!Where we're going, we don't need no buns.....
1950 GMC 450
1951 Chevy 1/2-Ton
The GreenMachine
In the Stovebolt Gallery