Strange that they all disappear at the same time being in two different data centers. This must be a DNS issue, are all the servers/sites using the same DNS server then? That may be the root of your problems, not the servers themselves, specially if your clients use those DNS servers also, or DNS servers whose domains use the one DNS server
Having double checked, they don't all use the same DNS servers, and all four boxes all stop one hop before they should on a traceroute. (I know the boxes respond to ICMP traceroute because from another isp, those last lines do resolve).
Whats even stranger, my isp is saying they can access all the sites without any problems, its just me!