It looks like you are on a shared hosting plan and your host limits the number of page requests per second. This is relatively uncommon, but not unheard of for some hosting providers. Unfortunately the admin loads some of the images dynamically with requests to a php page that returns the image thumbnail. Your hosting company sees these as separate page requests even though they all load on the same page and blocks page requests for several seconds after. This is the issue. You would have to talk to your hosting company to see if they can increase the limit or you may have to switch hosting providers and/or plans if they are unwilling to remove the restriction. Who is your hosting company? Just for reference I'd like to keep track of hosts that have this issue.