Headgaskets, timeserts, on a Northstar. Fun job eh?? :) BTDT

Anything at this point is speculation. Typically I advise to replace the plugs AND the wires together as a matched set. The lean fuel mixtures that engines run today cause a relatively high spark demand voltage. Any flaw in the insulation will tend to allow the spark to leak through. Even the action of putting the old wires back onto the new spark plugs can scratch the plugs porcelain surface and promote the creation of a carbon track. Once tracked, both plug and wire have to be replaced to solve the problem.

Due to the design, it's not impossible to test and prove the missfire in the shop as long as it happens often enough, however it is more difficult compared to any engine that has the better access to the plugs. The fact that this one still had plug wires and not the coil cassettes is in your favor. My first move would be to see if scan data shows missfire data. That takes an O.E. level tool BTW. If it does then it will help identify the cylinder that is missfiring. It gets much easier to concentrate on just one cylinder, VS the whole engine.

From there scoping secondary voltage demand, and spark voltage and duration would be next, as well as testing 4 gas concentrations out of the tailpipe, and I would be using my low amps current probe to watch ignition coil current, and the injector current and pintle operation.

i go to a caddy website and people have mentioned how fantastic their car runs after they put in a remaned motor or some such thing. that sounds sweet. my motor has 110k and ran great till i pulled it. i think it runs about the same. it was my wifes car and i rarely drove it. even now i am not sure it is really running right. i have heard its almost impossible for the timing chain to skip a tooth. there are 4 cam sprockets though.