Not sure I can help with the video monitor but we had same problem took us ages to work it out!
We bough Tommee monitors with sensor mat - settings were CRAP to put it short but went for the Angel care set again with mat lots more options on settings and have found the only one we need does not mess with the wireless which is the alarm goes off if she stops breathing other than that we can hear her from every room in the house lol
No idea if theres anything you can do to your wireless connection

- thats something we should look into because like you we're all wireless this seems to be the only way atm.