Incapaz de resolver problemas do apt-get Ubuntu 14.04

Question

Estou usando o Ubuntu 14.04. Quando eu estava usando uma conexão wifi tudo estava funcionando bem. Recentemente eu mudei para uma conexão com fio. A partir de então, quando tento atualizar e instalar pacotes através da linha de comando, estou enfrentando problemas. Quando eu sudo apt-get update , no final da saída está mostrando

Fetched 3,506 kB in 4min 48s (12.1 kB/s)                                       
W: GPG error: http://archive.canonical.com trusty Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <[email protected]>
W: Failed to fetch http://in.archive.ubuntu.com/ubuntu/dists/trusty/Release  Unable to find expected entry 'restricted/source/Sources' in Release file (Wrong sources.list entry or malformed file)

E: Some index files failed to download. They have been ignored, or old ones used instead.

E também quando tento instalar pacotes, por exemplo, r-base-core , está mostrando dependências não atendidas

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 r-base-core : Depends: libblas3 but it is not installable or
                        libblas.so.3 but it is not installable
               Depends: liblapack3 but it is not installable or
                        liblapack.so.3 but it is not installable
               Recommends: r-recommended but it is not going to be installed
               Recommends: r-base-dev but it is not going to be installed
               Recommends: r-doc-html but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Eu tentei todas as soluções a seguir, mas ainda assim o problema persiste. Eu não quero reinstalar o Ubuntu, muitos outros softwares foram instalados nele.

solution1 , solution2 , solution3 , solution4 .

Qualquer ajuda será muito apreciada.