When u dl using dcc, its ip to ip so its unconnected to the irc network your on, I hardly think that the ISP is monitoring what irc network your on then monitoring if you start a dl while on that network, using that network, and then waiting tell u reach 400mb and then stopping the download.
You never mentioned if you can resume these downloads at the 400mb mark or not.
Since you say it doesnt occur on other networks, maybe try this.
Start download, disconenct from irc server, connect to different irc server where it doesnt occur, see if download stops at 400mb mark.
*** beaware that some xdcc/fservs well halt your download should you leave the irc server (this is a scripted action), so this may not be possable to test ***
