It sounds like you are connecting and uploading to others just fine now, but you are getting "Transfer Interrupted" messages because you have a dynamic IP address. As its name suggests, the IP address you have changes every so often. The exact amount of time before your IP change changes is something only you (or somone else with similar service) can discover. Ask your DSL provider or consult documentation. It may be that every dynamically changing IP computer acts in much the same manner (in terms of frequency of changes), but I don't have dynamic DSL (or DSL, for that matter), so I could not say how often it occurs. Ask around. But from what you're telling me, it seems that people are downloading files from you fine. With the Force IP turned on, downloaders can "penetrate" the firewall without using largely unsuccessful push requests. Downloads go fine until they are interrupted your IP address change. The people downloading from you cannot find you (thus, can't resume from you) after it changes unless they search for the file again and attempt to resume from your new IP. Most people give up at this point, though.

This is a tedious solution (not really a solution, but a temporary fix to let quick uploads occur) and becomes more frustrating with time, but you can do this if you want to. Check periodically what your IP address is and change the LW settings to match the new IP. This will get old pretty fast, but I really have no other ideas on how to fix this. It sounds there is need for a feature in LW that automatically discovers and forces your IP address for you so that you don't have to do it manually. Remember, though, that this feature (and the temporary fix) would not solve the interrupted downloads, but it would allow your uploads to occur without having to worry about firewall limitations.
I guess for Gnutella, a dynamic IP address isn't exactly a good thing. Sorry I can't be of any more help! :-)