Apache/PHP get stuck when using form "POST".

Hi! I have an interesting problem. I am wondering if it is related to PHP or Apache. So, I need your feedback on this one. What happen is when I use the HTML Form "POST" and submitted it. It worked fine and getting to the 2nd page. The 2nd page include some PHP script and a modem script.. PHP is able to executed the script and get result from the modem. They all work fine. All of sudden I get customer complaining about problem with website. After some feedback from them, I use two seperate web browsers to browse my company's website. One for the modem and the other for browsing around. So, when I did the form "POST" and wait for the modem to fnish the task. On the 2nd browser, I was able to browse around but when the 2nd page from the 1st browser was executed. The 2nd browser, all of a sudden, it stopped browsing as if the webpage froze. I kept trying to browser around but with no luck until the modem from the 1st browser is finish. It is when I was again able to browse around on the 2nd browser. In my opinion, the problem is either related to the C programming for the modem or Apache that isn't able to dedicated it's resources to the browsers. That the problem have nothing to do with PHP itself. Am I right? Thanks, Scott