Erratic Masterpage cast exception

I have a masterpage named MasterpageController with a menu.  In the
codebehind I have a method SetMasterMenu(<some parms>) to display the
appropriate menu items.
The content page, which has a MasterType directive, executes the method
using "this.Master.SetMasterMenu(<some parms>)".  Runs fine for maybe 5-10
"start with/without debugging"'s, while I am in development mode in
vs.net  2005, and then I'll start getting an "Unable to cast object of type
'ASP.masterpagecontroller_master' to type
'ASP.masterpagecontroller_master'"  when the content page attempts to
execute the method. I have to restart IIS
to get the app to run correctly .. then a few starts later I get the
exception.
I havent seen this behavior when I publish the app onto a staging
server, just on my development laptop.
Any ideas on how to fix this?  Does it only happen while in development,
not in a published site?
Thanks, Mark

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