Hello,
Following customer reports, we investigated, the cause are prestashop servers since some resources needed to load the back office are unavailable more or less long and recurrent, therefore cause more or less severe problems
Sources involved :
http://profile.prestashop.com and http://gamification.prestashop.com/css/advices/advice-1.6.1.21_479.css
Solutions :
1. Disable module gamification
https://www.webbax.ch/2014/05/14/prestashop-tres-lent-brusquement/
2. Prevent check update modules by Administration/Preferences
3. Complain to Prestashop on their forum or technical helpdesk