When client's IP changes very frequently like say every 5-10 seconds, the FrontAccounting session Anti-Hijacking feature takes over and makes it impossible to retain a FA session for any practical length of time.

In this case, you can disable the check in includes/session.inc

Look for a function called preventHijacking() and comment out the lines

    if ($_SESSION['IPaddress'] != $_SERVER['REMOTE_ADDR'])
      return false;