Here's a thought that may be worth looking into.
I had a used HEI distributor on my SBC in my 69 Pickup. The centrifugal advance had rusted enough that the weights would stick at full advance. When the engine stopped they would reset. So when I checked timing in the driveway it was fine. When I drove it, it would ping heavily and idle poorly. I finally discovered the problem by revving the engine with the timing light connected. I could see the timing advance "snap" to 20 degrees at about 2500 rpm, then stay there as the engine idled down. I replaced the distributor with a new one but I probably could have cleaned up the advance mechanism and greased it to make the old one work. Might be worth pulling the rotor to take a look, Curt.
regards,
Leon
