It sounds like the wifi is going to sleep, causing the connection to drop, that's all I can think of. Does it still happen if you turn wifi off?
When you are on wifi and playing a stream it's going to be using your home internet, when the screen is turned off that connection is going to drop and the app will have to re-connect via 3G, I wonder if it's timing out when that happens. Have you waited a bit to see if it re-connects by itself?
The best test is to see if it drops when wifi is turned off.
Be careful with 3G though, unless you're on unlimited or a generous allowance as streaming can use a fair bit.