My car is always in the garage overnight, partly because of security but also it means I don't have to scrape the ice off the windshield every morning. My wife has the car every morning during the week and so I don't want her to have to clean the screen down now that it is winter. I can try leaving it out overnight at the weekend however.
The problem is that it is so random. Sometimes there will be a period of almost a week where it does it every day, then it won't do it for perhaps a month. The random nature makes it next to impossible for me to be able to demonstrate to anyone. When I do hit a period where it is doing it regularly, I will try to record a video of it doing it.
I have tried just letting the car idle and it does eventually return to normal revs. It has been a while since I tried this so I can't be certain but I recall that it takes in the order of three minutes. It is a similar time to what I might expect it to take from a normal cold start (with normal revs) to dropping down to normal idle when it is warm.
I think the best thing is for me to pay attention a little more when it happens again and try to keep a log of when it happened, the temperature reported by the car, the garage temperature (I will have to go and buy a cheap thermometer) and the outside temperature, etc.
Hopefully with it being winter, it will start to happen more often, although that is no guarantee as it can do it in Spring and the Fall.
Note, I picked up my car from having a minor service today. I asked them to watch out for the cold start problem, but of course it didnt do it. They did check the 'codes' and said that there was nothing in the log which indicated any problem. I don't know much about this though, so perhaps this sort of problem woud not result in a 'code'