losing session values

Hi all, We have developed a site on a server now the client has moved the site to another server. Both servers run apache. However some of the Session variables are losing their values or their values are getting changed. We have a number of header("location: xxx.php"); and they have the exit(); function after each statement but we are not sure whats causing the session problem. we also have a few javascript popup windows, is there anything that people can tell me from experience that might be causing these session variables to be altered?? thanks in advance Angelo