Check the tools->statistics->bandwidth->upstream

Keep an eye on the graph while someone tries getting an upload from you.

If they get the first few bytes at a nice high rate, but then nothing for 30 seconds or so, chances are your ISP is using filesharing controls.

Your settings are probably fine, so something else is interfering. Monitoring the stats for upload bandwidth should give you a better idea what is happening. Grab screenshots of patterns--they can be useful when explaining the problem to the ISP techs or others.