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Misfire from cold start

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hi everyone I have a ford Sierra Cosworth. 4 cylinder 16v engine. Siemens dekka 80lb injectors coil on plug conversion ngk iridium plugs running a link monsoon ecu. The car runs lovely nice and smooth no flat spots no issues at all. Until you part the car up and leave it over night. Cold start and there is a misfire. Give it a little Rev and it still misfires. Quite a prominent misfire too. For a couple of minutes and then it clears and is fine! Could a too cold plugs cause this ? Or maybe a leaky injector ? Injectors are new so I’m leaning away from this. Any ideas I would appreciate it. Thanks.

Figure out which cylinder is causing the misfire, by removing the connector for the coil-on-plug. First, look for a wiring/ connection issue with the coil connection and the injector connection. Does the misfire cure with wiggling the wires, or disconnecting/reconnecting the connector. If so, clean the contacts, add dielectric grease to the connectors and see if the problems remains cured.

If that hasn't helped, then try swapping the coil with another cylinder and see if it moves to the cylinder with the original coil. If so, then replace that coil. If it remains the same cylinder, then try swapping the fuel injectors. Again, if it follows the injector, replace it.

Oh, while swapping the coil, remove the spark plug to see if it is fouled. This usually isn't a problem with iridium plugs, but my first thought was a fouled plug.

If none of the above and it only misses after being parked for a while you could have a bit of coolant leaking into one cylinder from a dying head gasket. Does it blow smoke or a lot of fuel vapor on start?

Will pull the coils 1 at a time when cold to see if it a plug / coil.

Engine is pretty freshly built only done a few hundred miles. And yes it was built properly also.

My initial thought was a fouled plug. I will get looking from a cold start.