iMac (OSX 10.11) freezing when scrolling (Safari/Facebook)

The last couple of weeks, my iMac (OSX 10.11) has been freezing while I've been scrolling on Facebook (Safari). I've contacted Apple Chat 2 times about this matter. First one useless. Second time rep had me reset PRAM. That seemed to work, but then didn't work. I've tried to contact Apple Support the last 2 days (Mar 17 & 18) and cannot do it. Both Chat & Telephone. I get to entering my serial number, click & get "cannot connect." Is there something wrong at Apple's end or is it my computer?