After changing from LW free to Pro, I started having the same problems as you with the following exeptions:
-right after starting Limewire, the download speed would be good for about 15 seconds and after that it drops to 0-5 kb/s.
-every now and then during downloading, the speed goes up again for a few seconds, even up to outrageous speeds like 1500kb/s
-the upload speed is fine all the time 40-80kb/s even when download speed is at 1 kb/s

NOW hows that.

After going thru this forum and fixing all the known problems, now I´m getting bursts of 350kb/s download speed every once in a while for a few minutes at a time, but mostly the speed is at 0-10kb/s

Just started up LW and again for the first 20 seconds the dl-speed tried to rise every now and then and got up to 30kb/s for about 3 seconds at a time from where back down to 0-3kb/s to idle. Uploaders are still getting good speed all the time and continuously 40-80 kb/s