Dial timeout on isdn router with ip-tables logging

Hi, I'm running a self-made linux-isdn-router at home and wondering that the idle timeout isn't working correctly. I see several inbound connections ( in the fw-log ) which will be dropped after logging. now, usually the idel-timeout will wait for 300 seconds until no traffic was gone through the isdn-interface. I am logging Portscans, etc. on the isdn-interface - could this be the reason why the idle timeout won't work correctly ? I didn't try to not log incoming syn's on the isdn-interface ( it's very important for me to see - who wants to come in .. ). What should / could I else do to prevent that the timeout won't be reached ? yours, bruno