How to *diagnose* windows explorer frequent freezes

Just use ProcMon (Process Monitor) to examine the events. Filter by explorer.exe and then category network. A quote from Mark Russinovich:

When troubleshooting a hang with Process Monitor, you should first see if there are any gaps in operation time stamps that match the hang duration. You can look for lengthy operations by adding the Duration column to the display and then making sure to filter out operations that commonly don’t immediately complete, like directory change notifications. That can be useful when you don’t see a significant time gap between operations because the process has multiple threads, some of which continue to operate while the one causing the hang is dormant.

• http://blogs.technet.com/b/markrussinovich/archive/2010/01/13/3305263.aspx

I think your best methodology is to take a tool like Process Explorer and try to link the freezes to a particular process. That is, what process is on the CPU when the problem happens? You should start by clearing out as many non-essential processes as possible and have your system running lean (perhaps even safe mode, no network or external drives) and see if the problem still happens. If it does not, then you can add back processes one at a time and see if you can isolate the culprit. If it happens in safe/lean mode, you have a deeper problem and can start with the things that are actually running in that mode.

Per comment: You could use perfmon and add CPU and Network usage to be tracked and have perfmon log over time. That way, when you have a freeze, you could go back and look through the perfmon logs at the time and see if you can correlate anything.