Another thing to check is the plug wire connection at the plug. Most aftermarket wires, regardless of price, simply use a round crimped metal connector that after a few plug changes no longer has good tension against the plug. Easy to test for this, assuming your plug boots aren't frozen to the plugs, just wiggle the plug wire terminal at the spark plug. If you can feel that is not holding the plug tightly, that may be a big part of your problem. AC/Delco wires used a metal clamp with another clamp around the outside to maintain tension on the plug. They were the only brand of plug wires I'd ever use in my old '77 Suburban.
