So you have already set our upload to 80% of the up capacity of your line? You shouldn't set it higher that that, even if you're not surfing or anything, as it could also slow down your downloads, especially if you use a release version of amule which doesn't have UBT...
On a 256 kbit/s (32 kbyte/s) an upload of 25kbyte/s should be ok, you can experiment a little with that...