Best Way to Test a Site

Hello, I directory where people pay per listing.  Similar to healthprofs.com, I am having issues w/ the registration and some other minor issues.  the problems are occur on very rare instances w/ different browsers and addon combinations that are effecting it.  I can not recreate the issue, but I know it's a problem.  I am capturing as much as I can about the people that sign up and can't get through.  When they call in I get their name and check what version of what browser, how far they got, os, etc, still no commonalities so it's really frustrating...

I would really appreciate any suggests to test the site, programs, ideas, services, 3rd party companies, etc

Thanks.