Skipping at idle - Taurus

I have a '98 Taurus 24 value DOHC with 100K miles. Changed plugs a month ago and changed fuel filter, added gas treatment but now have an infrequent skip
when starting up. It usually goes away after driving 15 miles or so but always comes back after the car sits and cools down. Any ideas if it is fuel related or spark related ? It is not bad (feels like the engine dies for a fraction of a second) but I'm concerned it may die for good in the middle of nowhere. I've thought about a bad injector or coil breakdown but since it comes and goes don't know how to test for those. No OBDII codes have shown up.

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