stab in the dark here, might be todo with max connections,
try:
"upload limit" at 75-90% of your "real" capacity
"max connections" 200
"max new connection / 5 secs" 10
and no more then 3000 overall download sources, if more, stop downloads to get it lower then 3000
i personally use even lower settings, not because i have a crappy modem, but i think lower settings equal lower overheads, equals a more stable connection for longer.