Well, after many months of pulling my hair out, we found the problem causing intermittent misfires and poor running. The connectors from the wiring harness to the coil packs.... apparently, the quality of the connection between the plug on the harness and the pins on the coil, degrades after some time. This car is a 100% track car, and has seen almost 20K track miles. Still has the OE harness. When the car begins to run poorly, we've been able to repeat the fix at the track a few times: Remove each COP connector from the coil pack, carefully bend all of the silver tabs (2 per connector, using a pic) to make better contact with the pins on the coils, carefully replace each connector. These are really fussy, little delicate things. All I can assume is that heat/cold cycles and track duty begin to fatigue these things. Next purchase is a new OE harness....I guess. I can't find anyone that knows of an upgrade for this. If you do .... Please Post!! BTW - months ago when this all started, I had Mustang "gurus" tell me that my motor was toast. It's not, we checked everything, including doing a leak down test and all is great. That, and combined with the fact that when the connectors do their job correctly, the car runs like champ. Anyone else have this issue??