If that is after a flush.
I would make sure your engine ground is good. It is located at the back of the passenger side head and connects to the firewall. It needs to have good metal to metal contact. So you may need to remove a little bit of paint.
I use a little dielectric grease between the ground wire, head, and firewall, it seems to keep rust away. I also do not use the stock cable. I make a cable out of black battery cable.
Anything magical about having the ground at the back of the head? I have a ground strap from the engine block to the frame at the front. Nothing really going from the head to the firewall.
1965 Coupe 5.0 GT40 Intake and Heads, 65MM TB, chipped, T5, Shelby drop, roller idler, subframe connectors, Magnaflows, Hurst shifter, MS clutch, BFGs on 15" TTDs, Roller perches, Bilsteins, 3.54 gears, LS, Needs everything else...
"If you didn't build it; it's not really yours."