iPhone 4s reverts to 4G despite being connected to WIFI

I am having a situation I was wondering if others might be experiencing. At home I am using a Linksys Router running a variant of the Tomato firmware which allows me to monitor the amount of traffic coming and going on our internet service. If I am streaming a 128k radio station, I see a nice steady stream of data being downloaded by the phone, but then at some point, randomly triggered, the app will pause to rebuffer the stream. After a few seconds the stream continues to play BUT the monitor graph on the router drops off towards zero. The stream will continue playing but the data is NOT coming from the wifi router. The router did not experience any issues because other devices on the network continue with incident.

I have gotten into the habit of when I get home, I turn off carrier data. The stream will play the entire length of the hockey game without a skip, but when I enable carrier data, at some point during the game, the stream will pause, rebuffer and continue using carrier data and not wifi data.

Anyone experience something like this?

Thanks for your time.