security trimming does not work

Hello All.
I have a Website built on .NET with the following specificatoin:
- dot net framework 2.0.
- windows server 2003.
- IIS6 .
- Used Role Provider and sitemap to generate my main menu for each user Role .
- Used web.config files in each subfolder to secure access to pages acording to users roles.

Every thing was working fine ,until I Upgraded  from framework 2.0 to framework 4.0 , i faced two problems that appears sometimes :
1 - security trimming does not work, such that all sitemap nodes are apears for all users regradless the node in their role or not. 
2- when user A request information he gots information requested by User B.
Notes:
* after restart IIS (some time the server machin) every thing works fine.
* I notced that this problem ocurres when there is alot of requests come to the server .
* the s ...