Hanging requests executing

Dear reader,

We have two Win2003 webfarms each consisting of 5 servers. Which we call
cluster1 and cluster2. Cluster2 is the bussiest of these two and periodicaly
(varies from 2 times per day to two times per week) one of the servers (this
is pretty random) starts to build up Requests Executing. During this
'queueing' it still serves about approx. 75% of the requests. The requests
that don't come back with an answer are added to the Requests Executing
count. This goes on until 100 requests are queued then it starts throwing
errors like: "The CreateObject of '(null)' caused exception C0000005"

To me it seems like one of the 4 worker processes is not responding anymore.
Is there a way to see why this is happening?

SQL server doesn't report any locks.

Thanks in advance,
Jorre