This method worked for me.....just have a try...
check the file /etc/apt/apt.conf
The contents were,
Acquire::http::proxy "http://<proxy>:<port>/";
Acquire::ftp::proxy "ftp://<proxy>:<port>/";
Acquire::https::proxy "https://<proxy>:<port>/";
This was the reason why you could reach proxy but couldn't get past it, since there is no username password information. So just put that info into it..
Acquire::http::proxy "http://<username>:<password>@<proxy>:<port>/";
Acquire::ftp::proxy "ftp://<username>:<password>@<proxy>:<port>/";
Acquire::https::proxy "https://<username>:<password>@<proxy>:<port>/";
save the file and you are done...
BROTIP: More better add these lines in another file, /etc/apt/apt.conf.d/80proxy
. This will ensure that after a version upgrade changes won't be lost.