Apache cannot display PHP page

Hi, I'm currently using Apache1.3.22/ApacheJServ1.1.2/PHP4.3.2 and it's working fine until now. Now, my PHP engine seems not working sometimes. Let's say sometime page A which contain php source running well and sometimes don't. For example I have this simple code : <html> <body> <? echo "hai"; ?> </body> </html> Well... when i call this code sometimes it appear correctly... in my browser it display : hai But sometimes it appear blank. And when I look at the source (in IE I choose View => Source) of the page it display just the same as the original code. It's something wrong with the Apache or what ? Please help me... Thanks, Feris