More on this, The idle isnt rough as in I have a set of very aggressive cams but its more like it'll dip to around 500rpms and then go back to normal idle around 1000
I consider my car to run extremely well even though it occasionally does the exact same thing with a bouncing Idle.
After I disconnect the battery the ECU has to relearn the car and the idle is extremely rough for about 100 miles of driving.
I had the same sort of thing happen two days ago after the same weather conditions you experienced. It was really cold out after a week of warm weather and the car started normally and idled for about a minute then dropped to 500,.. bounced around for a few seconds then went back to about 1200 rpm.
I figure as the ECU learns the car, it uses that information at the next startup, and being is it was much colder out the switching point to start turning off the "choke" (reduce the idle on our fuel injected cars) was different and didn't "take", so the car waited longer before reducing the idle.
The ECU reacts instantly to some sensor data information but needs much more time to assess other data,.. let's see if my car "remembers" how to warm itself up next time I start it.
Your car may very well have more going on then just its normal quirkiness but if you're doing mainly short trips in this cold weather without the car really ever warming up fully, it's a good idea to give it a hard highway run for about half an hour every week or so.
( both my sisters phoned me to stay their engine light came on,... a half hour on the highway cleared the code both those times)
PS... proper idle is 720 rpm (when fully warmed up),... perhaps your car keeps trying to bring it down but it doesn't "take" (for whatever reason) so it needs to give up and go back to 1000,... then it tries again later.