about Oracle logon

The system I am running include PHP4+Oracle 8i(8.1.5.)+Slackware7.0+Apache web server 1.3..All of them are installed in a same computer.I wrote a php program to connect to Oracle Database using PHP OCI8 function and run it as a cgi program.Now there is a problem for logon the database.Firstly,I am able to logon the database successfully,but it become unstable to logon the database after logon a several times.Then,sometimes logon successfully or sometimes don't.Finally,I even couldn't logon again.It only works again until I restart Apache web server.at the same time,I have found there are a lot of threads for oracle logon in memory,which aren't killed before the web server is restarted.Is there any configure I need caution?Do you have any suggestion?I hope get your help. Thanks Steven.