Chris has missed the point...
The problem isn't that the report takes 4 minutes to run, (and it's highly unlikely that UTF-8 to UTF-16 conversion is adding any substantial delay to the report process) - it's that the idle timer is expiring in the middle of that long report being processed.
Windows is queueing up the application idle event, and it's getting processed as soon as the application regains control. You don't want the processing time for the report to "count" against the 2 allotted minutes for sitting idle, since there's really nothing else for the user to do until that report finishes.
The easiest option is simply to disable the idle timer prior to running "long" reports, then re-enable it once the report finishes. Then you can start the process of making your reports run faster - probably by tuning the SQL of the report query.
-Paul Horan-