Not All IPs At Equal Speed On Our Rack

Where I'm certain that this is a gateway issue, perhaps experience will prevail, where someone having experienced a similar circumstance will have a solution.

We have a box, running CentOS 4.3 (64 bit) that has an 8-block at LayeredTech through ZipServers. Strangely enough, two of the IPs are operating at a reduced speed!

To test the matter, we've posted a file shared across all IPs (/var/www/html/testfile.gz), and when downloading through different IPs, the speeds are different.

For example (fictitious file links):

72.232.186.42/testfile.gz (DL at 350k/sec)
72.232.186.43/testfile.gz (DL at 350k/sec)
72.343.186.44/testfile.gz (DL at 350k/sec)
72.232.186.45/testfile.gz (DL at 70k/sec)
72.232.186.46/testfile.gz (DL at 70k/sec)

Further, this isn't related to Apache, as FTP is affected in the exact same manner. If we FTP into the machine using a same account, but with different IPs as hostnames, the download speeds differ just as the HTTP downloads do.