I am not trying to sound like a smart-ass nor am I claiming to be an expert on this matter but could it be because your PC is trying to connect to the AP it is receiving the highest signal strength and seeing how it is in the middle of two AP's it keeps bouncing back and forth between the two, one being encrypted and one being open?

I am attaching a "paper" titled "Routing and Channel Assignment in Multi-Channel Multi-Hop Wireless Networks with Single NIC Devices" by one Jungmin So and a Nitin H. Viada from the University of Illinois at Urbana-Champaign, that I found a few months back, I can't even remember what led me to find it, I think trying to find out if the signal comming out of an AP would constitute as a burst transmission or streaming, that I was surprised that I still had. It doesn't go into mixing open and encypted signals, and after about 5 pages I have no clue of what they are talking about, but maybe it could be of some use to you?

Cheers