I'm on PR1.3 and I don't have this wi-fi re-connection problem. My N9 will kick out my 3G when I'm in the range of a known wi-fi as it should. I don't think PR1.3 is causing the problem.

However you could try the following:
1) use static IP. If you need more detail see post #7
2) change your wi-fi channel.
3) change your router. The brand may be incompatible with N9. I'm using TP-Link and it works great.

One more thing. Your wi-fi network is set to "automatically used network isn't it? Just checking. Allow background connection should also be enabled.

I have a feeling No. 1) may solve the problem but you must be willing to try it.