Weird Network Cable Unplug Problem

[Rhombuss]

Noticed this for a while, but never got around to figuring it out. My HTPC computer is hooked up with a 100TX network connection, but whenever I try to transfer files to it, it's extremely slow, around 500 kbps as opposed to a 10-15 Mbps. But once I unplug the network cable from the HTPC comp to the router, then plug it back in, it begins to transfer at around the 10-15 Mbps.

What could the problem be? This problem arises everytime I turn the computer on or restart (I think). It's fine for normal network use (internet browsing), but once I need to transfer files, I always have to unplug/re-plug.

Thanks in advance!