I too experienced this irritating problem, a while back.  After much searching, with no answers found, I contacted my host's support.  They moved me to a "less-used" server and the problem went away!  It must have to do with being on a shared host, and the number of times IIS is being reset by the actions of other websites.

You can ask your host, and if they're nice (I left two other hosts for lack of support for DNN) they just might do the same for you.  I'm very happy with the support I get from WH4L, and my spontaneous logouts are just a bad memory.

kfayal, if it never happens when you're logged on as the superuser, I would guess the issue is a page is attempting to access a resource outside the portal, and getting automatically logged out i.e. DNN detects that you've switched portals, and requires the new user to log in except if you're a superuser, as they have access automatically. If you could install a http proxy such as fiddler http://www.fiddlertool.com/fiddler/ , then you could browse as a normal user, and when you're logged out, look back through the http session list to see which request(s) are outside the portal e.g. mysite.com/portals/1/somefile.jpg followed by mysite.com/portals/2/someotherfile.jpg would show a switch from portal 1 to 2.

Cathal