Erratic report execution

When we request reports through the Report Manager, sometimes they run
and sometimes they don't (ie they time out after 30 minutes.)
Specifically, when I run exactly the same report with exactly the same
parameters and examine the ExecutionLog I will see numbers such as:
First report request (does not run):
TimeEnd - TimeStart = 30:14.4
TimeDataRetrieval = 0
TimeProcessing = 1814137
Status = rsProcessingAborted
Second report request (does run):
TimeEnd - TimeStart = 00:27.7
TimeDataRetrieval = 14087
TimeProcessing = 12502
Status = rsSuccess
This situation occurs with different reports (not just one specific
one), different users (not just one specific user) and unpredictably (I
have not been able to define the conditions under which a report will
not run.) Sometimes running a second request immediately after the
first will appear to "trigger" the report to run (which made me suspect
a cache issue) but at other times the second request is made a day or
two later and the report runs correctly (ie long after the cache has
been flushed.) More often than not the reports do run.
We are running SP1 and all of the reports use multiple datasets via
stored procedures. Most of the stored procedures use (NOLOCK) on the
SELECT statements.
I searched the forum but could not find a discussion on this topic.
Does anyone have any suggestions as to how I can track this down?
Regards.I am also facing the same problem. In my case, the details when the report
didn't run are:
TimeDataRetrieval = 0
TimeProcessing = 29305548
Status = rsProcessingAborted
Can someone prescribe further course of action?
Thanks in advance!