Idle lag/stumble

So I had an interesting event yesterday. After work I went to get a carwash, it's too cold here to wash by hand still, and right before the dryers kicked in my car started to lose idle RMPs. I normally idle around that 1000-900 area but the car dropped to 500 and came back up, back down and back up and down once more to where I thought it would stall out. I had it in neutral and was just sitting there at the time. When the door went up I didn't even dry the car, I just went to the parking lot and sat. I did manage to catch it doing it again. Went from 900 to 500 back up then down to 700 and came back. I decided I needed to get it home and it hasn't happened again.

Anyone else have anything similar happen? The closest thing I could find was after filling up with gas but I didn't fill up yesterday. This morning on cold start it ran fine but I did notice when I got to work it seemed to idle low and was around that 700-800 range.