What components are involved? (load balancers, switches, desktops, routers, and/or servers)
What's common to this problem?
Does it happen from everywhere on the network?
Do all systems and/or OSs seem to be affected by this problem? For example, turning ECN on under Linux can make it hard for a Linux box to connect to some systems that drop packets with "undefined" header options. Because ECN is an optional setting not defined in earlier RFCs, some paranoid firewalls and routers will assume the packet is bogus and drop it. In these cases, only the Linux systems with ECN turned on would be experiencing network problems, whereas all other systems would be functioning correctly. The point is to determine the scope of the problem rather than assuming it's universal.