Unresponsive System For a Few Minutes

We've been seeing a lot of this lately and I need some suggestions on where
to start with diagnosing this problem.
We are a domain node on a WAN and several of my users, including myself,
have indicated that periodically their systems will simply become
non-responsive for 2-4 minutes all the while they are clicking stuff and
nothing is happening, after the 2-4 minutes have passed, everything they
clicked, all opens at once and the system is then again responsive. There is
no disc activity during this period of unresponsiveness; the system is simply
dead until it comes around again.

Nothing in the event viewer indicating any problems has taken place.

Any ideas where to start?

TIA