do you use winmx or other file sharing software?
when someone does a search for a file, you pop-up as having it, and then they choose you to D/L from, that their computer will check periodically to see if you have an open slot to let it start d/l'ing. unless they do a new search and cancel your file from their list it will keep checking your conection to see if you are ready to give up the file.
this continues even after you shut down the sharing software. and if you have a static IP it would continue even after re-boot. up until the other person decides to refresh his search and try a different source (and many of the ones who download alot of files don't go through and delete the 'dead links' very often.
I'm by no means an expert but this is how it was explained to me.
it may be completely unrelated to what you have going on, just a thought, but if you are sharing files, all those pings could be demanding a lot of attention from your firewall.
you may be able to check it by turning on your file sharing software when you are having this problem and see if it goes away (assuming you have set the firewall to allow transfers through this program, it should stop being seen as an attack if the software is running and able to recieve the request.)
there are a bunch of guys here who know more about this than me. for those of you who do know more, am I correct on this possability, or am I missing something?