So you have used a www-browser to download? Browsers often show false downloads speeds, for example 1000MB/s which soon drops down to the real download speed.
Try this to see if you really do have inconsistent download speed:
time wget http://www.site_near_me.com/a_very_large_file.tar.gz
The "time" -command will show how many seconds it took to download that file. You can also watch wget's download speed to see if it fluctuates. Try this a couple of times and with different sites to see if you really have a problem.
I have not tumbled into any real transfer speed problems with any network card in linux.