Can't connect with Remote Desktop anymore

Hi I'm running a remote desktop on my windows 7 Ultimate 64-bit system. At first, I can connect with it but after a few hours I can't anymore... not by using the dyndns or the external ip... I have routed it correctly otherwise I wouldn't be able to connect the first time but it's just suddenly that it shuts down... The Windows 7 is connected with wireless to my network, could that be the problem maybe? I'm really out of options to try. Have this problem for a few days and well it always works for just a few hours. No limitations to the Remote Desktop hours or anything...

Thanks