I don't know if this helps, but about a year and a half ago I put in new Autolite plugs with FMS wires.
The car would shudder under heavy acceleration. It even sounded "wrong".
I thought it was the plugs and pulled those and put OEM back in. Car still ran badly. I checked the FMS wires and one seemed slightly "loose". Like you, I couldn't get the plug the "snap" in like the rest of them.
I put the Autolites back in and tried the OEM wires and the car ran fine.
I ended up trashing those wires.
Have you tried swapping out all of the FMS wires with the OEM wires?
Hope this helps somewhat.
