Unable to connect to server...intermittently

[AnnieB]

A few days ago I installed Linux Mint 18.2 Sonya Mate edition from the CD. I've been using Linux Mint 17 Qiana since 2014 and before that Julia and Olivia. The main reason I upgraded to Sonya was because for a few months now Thunderbird has been intermittently 'unable to connect to server' and I thought it was 'broken' on Qiana. However the same thing is happening on 18.2 Sonya. Firefox can also be very slow to find the home page and frequently can't connect. Also more often than not when trying to download from the repository there is a message saying "could not download all repositories'. Then, occasionally, everything works just as it should. I have had a lengthy conversation with my internet service provider and there seems to be nothing they can do....my smart phone email & internet connections work fine using the same wifi. Could it be the hard drive? Although I've been happily using Linux Mint since 2011 I'm still a newbie....not techo enough to know which server a programme is unable to connect to. Would really appreciate some help here. Thank you.