Your uploads shouldn't have bad effects on your downlding unless you're using a DSL type of service where upload & downld bandwith is the same entity. If that's the case, then reduce upload bandwidth to about 80% to see if that makes any difference. If it does, then you may want to reduce it a little further.

If that's not the case, then try deleting your LW Preferences folder whilst LW is closed; instructions here: Point #2 in Fixes for Limewire (sample at bottom of page.)

Not sure what you mean by error messages. What type of error messages? Are they firewall warnings or java errors or ... ?