Its usually to do with RAM being used up. Bandwidth is your connection speed ie 512k, 1MB, 2MB etc etc. Most important for PES5 is your UPLOAD speed. Which is usually 256k. But apparantly 128k will suffice for the game. When your opponent is hosting, you are downloading off their computer. But you can only download as fast as their upload speed. ie you can only take however much they can give at a time. If your lucky enough to find anyone with a higher upload than 256k..then your laughing. There should be no lag with them.
The lack of RAM will cause the stuttering in the game. Your bandwidth (internet connection) collects the data..the processer works with it..then sticks it in the RAM so you can see on your screen. This is happening millions of times a second...sometimes there'll be too much data for the RAM to hold so it'l skip sections of the game because it cant put it on the screen, which makes it look like its stuttering. If you run background programs like MSN, AOL, ICQ, Kazaa, MS Word, Notepad etc etc anything..it takes up RAM. All the internet based programs (MSN, AOL, Internet Explorer, Firefox) will suck up bandwidth as well as RAM. So close ALL programs before running the game to conserve RAM and, at times, bandwidth.
/semi-rant