RFI
Problem now resolved, but perhaps a post here might assist anyone else suffering a similar problem.
I was having severe problems connecting to various sites, the progress line would just lock up part way, or vids would stop at a random point.
The timing was so intermittent that I suspected RFI from somewhere, tried resetting my router wifi auto channel ad infinitum, or at least until it got fed up and locked me out.
Finally I nailed it - the radio dongle which connects keyboard and mouse was too close. It is now mounted on a six inch extension cable and this has solved the problem.
The cause was right under my nose!
So in the event of any wifi probs, first look to see if there is a wireless device which can be moved. A few inches may be sufficient.