Network Service Order keeps changing on me. Please help.

I have an iMac8,1 running Snow Leopard 10.6.2. Ever since upgrading to Snow Leopard, I have been having a problem with my Network Service Order changing on me and thus causing my Internet access to cease.

I have an internal network of PC and Mac computers connected to a Linksys wireless router. My iMac is connected to this network by Ethernet. For Internet access, I connect wirelessly to my neighbors wireless network using the built-in AirPort. I have figured out that in order for this to work correctly, the Airport MUST be listed above the Ethernet connection in the Network applet under Network Service Order.

This setting gets changed by the system frequently and without any reason that I can determine. I can set it back and resume my Internet browsing and within a minute or two, it can go right back to where Airport is listed below Ethernet. I will change it back and lock it down, and again within minutes, it will change back. Sometimes I will change it back again and it will work for hours or for the rest of the day. But without fail, it will always revert back within 36 hours or so.

Please provide me with some guidance as to how to resolve this matter. I cannot pinpoint it to this, but it probably is worth noting that I have Missing Sync for Windows Mobile installed on this computer which configures its own network connection for my WinMobile phone. I very infrequently connect my phone to the iMac, so I cannot pin the cause on this.

Please hep me! This is driving me nuts.