Strange connection problem...

Hello Folks!
I am new to this site and I want to thank in advance for any help that I may receive.
I have a Dell computer that came with Vista installed (unfortunately).
I spend much time online (I am a student and disabled) using the web for research, eMule to download files and Skype, yahoo messenger and MSN Live Messenger for chats.
I had a problem (lowid) with eMule and, thanks from information found on another site (http://portforward.com) I solved it by setting a static IP address on my pc and port-forwarding.
Then the problem began...
Since I set port-forwarding (that I don't want to unset) I experience this situation: from time to time, especially after rebooting or a long inactivity, the internet connection drops, but ONLY for some programs! While eMule and Skype keep working perfectly fine, yahoo, MSN and the browser cannot reach the net. I checked already the firewall (Comodo) and I can exclude any fault in it (the same happens if I disable the firewall completely). I tried to set port-forwarding also for HTTP, but it does not help.
Basically, the only thing that I can do in those situation is to restart several times the pc and the modem. After 3-4-5 attempts, finally all the programs that connect to the internet start working again.
Any idea about what is going on? Any suggestion?
Thank you again for your attention and your help!

Francesco
England