Well, true, I let that ultrapeer choice on, since it seems to help some others, and I didn't know it could affect that much the performance from my client... in fact it didn't, up to recently... Reading your post, I did just go to LW prefs and checked the "remove ultrapeer ability" as trial... on the spot, my client passed from good connection (it was meandring from good to fair and vice versa) to turbo charged... I hope it'll stay like that...
Thanks anyway for the tip