I agree these sites are all slow - and almost respond as if they are missing content even just the image spins a little bit before loading.....
1) Did this issue just start happening for existing sites or did you just start having this issue?
2) Do you have any other sites on this server that are working fine?
3) The next thing I would try to do is point the domains to a different IP address so that you don't have network traffic to the server and see if it is responsive
4) If you are able to get the server to respond after adjusting the domains, I would move each domain back one at a time.
This could be an issue with a script on a site, it could be an issue with traffic on one of the domains and overloading the server, or it could be something completely different
Do you have another server you can bring one of the sites up on to see if you get the same results.
There are many different variables here and I'm just trying to help isolate what / where the issue is.
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