Browser Compatibility Issue

We are unsure if this is Browser Compatibility or a different issue? The only issue we have been having from our customers is a LOG IN / LOG OUT issue. I am not sure which browser is causing this. I am guessing its Internet Explorer. Here are some replies from our customers... ..."How come I am being logged out every time I go to check a message or practically anything I click on, even to edit my own profile?" ..."Every single time I try to do almost anything on this site, it forces me to log in again. And I never get to do almost everything I'm trying to do, because after I log in again and try to click on something, it AGAIN makes me log in." We believe they are mostly using Internet Explorer (and maybe other browsers as well), where they are having these issues. We need our site to function properly across ALL browsers, including but not limited to: IE, Firefox, Safari, Opera, etc. We need you to diagnose and let us know the issues, and then fix it. Our site is an ADULT personals site. Please do not bid if you have issues with Adult Material.