Archive DL problems from File Sharing Host

I own two internet connections, PTCL broadband and Wateen Wimax. I have been having some strange issues while downloading rar archives from file sharing host like rapidshare, hotfile, fileserve e.t.c. using ptcl broadband. The problem I get is that even though the file is fully downloaded (as the actual file sizes and the DL size completely match), but when I extract them, I always get a warning message that certain archives are corrupt. So I have to DL those archives again to successfully extract the contents (Its happens with every batch of archives I DL). This issue is only with ptcl broadband. I never ever had these issues with wateen wimax. I had a friend who faced similar issues while DL rar archives from file sharing host. Did anyone in this community face similar problems with ptcl broadband? Also and more importantly does anybody know what is the reason for this and how this can be resolved.

P.S PTCL gives better speed then Wateen so thats why I prefer ptcl over wateen.