I am experiencing this problem as well.

There is actually a bug in Xolox that disregards the 'Use maximum upload settings...' in the preferences settings.

As a result, if your computer remains inactive for 15mn, the upload speed is automatically set to the maximum even if you've set a limit of 3KB/s.

If you have several uploads going, it eats up all your available bandwidth and leaves you with no download bandwidth available if you're on a cable modem.