I’ve had that exact issue before and it’s the ecu not having a good ground. Like cab said, resistance is getting higher at low rpm.

Two ways to attack. One is cleaning and sanding the mounting points on the back of the ecu and also the firewall so it has no paint or rust preventing the ground. Other thing to do is to run a dedicated ground to it. I prefer that method because it eliminates the problem for good as it can always corrode again and leave you stranded. As tjp said, grounds are always a good place to look.

As cab said wiring can be an Achilles heel on these things. Make sure you’re factory ground wire is in good shape. Adding a ground never hurts on these cars. Overall things just work better. Last 69 rr I had had the best electrics of any of my cars by the time I sold it. Extra grounds, bigger factory alt with dual fields, electronic regulator, fresh under hood harness from year one that included the late model stuff added like the regulator and ecu, cleaned bulkhead, added volt gauge and bypassed the amp gauge. Flawless.