我无法安装一些npm模块。某些模块正常安装，其他模块出现以下错误。
我没有使用任何代理。
这是错误消息：
npm ERR! Darwin 15.5.0
npm ERR! argv "/usr/local/bin/node" "/usr/local/bin/npm" "install" "-g" "express"
npm ERR! node v6.2.0
npm ERR! npm v3.8.9
npm ERR! code ETIMEDOUT
npm ERR! errno ETIMEDOUT
npm ERR! syscall connect
npm ERR! network connect ETIMEDOUT 185.31.**.***80
npm ERR! network This is most likely not a problem with npm itself
npm ERR! network and is related to network connectivity.
npm ERR! network In most cases you are behind a proxy or have bad network settings.
npm ERR! network
npm ERR! network If you are behind a proxy, please make sure that the
npm ERR! network 'proxy' config is set properly. See: 'npm help config'