Wifi connected but not functioning correctly iPhone 4S

Ok I have a iPhone 4S, when I brought it after Christmas it connected to my 'Virgin Media wifi' instantly and game me the full 60 meg speed. As time has gone by, and multiple wifi routers replaced (thought it was initially the router) the iPhone is now struggling to connect and when it does its speed is less than using and EDGE conection. Of I reset the conection settings it works well again but after a week or so it will deteriorate back to how it was before he restart. I'm not sure if this is an ongoing problem with the hardware or if the software doesn't like virgin media as all other makes of phone and laptop will happily connect to my wifi with decent speeds. Hope someone can give me a few pointers on what happens

P.S. I have restarted the router multiple times