Those VMS wires are not exaclty great either. NGK blue's are king in both durability and spark performance, even when compared to some of the odl school big brands that used to offer products for these cars.
Heck, I have trouble with a timing light sometimes getting a signal, that is how good the shielding is.
Is it hiding under the edge of the valvecover where the camera couldnt see in your last engine picture?
Just hoping to help you out. Ive dealt with frustratingly odd issues in teh past with the incorrect sparkplugs. Various results from fouling plugs too easily in cold starts, to hot weather pinging at random intervals (some days clean, some days noisy)