Bon, ok, there is another possibility.... You have a poor (read shitty modem) that doesn't like when you are running as an ultrapeer (too much connections). See under the statistics (in the tool menu) - network - the number under "active connections" should be under 6, if it says around 30 connections , you are an UP, you can change that in options - speed -desactivate UP capacity- check the box and click apply...

Bonne chance