Website stalling out "getting stuck" for no aparent reason?

Experts,

My question may seem vague but, I am really stuck and don't know how to proceed.

I've been developing a site for a few months now and just as of the last few days have run into this problem.  The problem is that when using my site (which has been developed in php) it gets hung up when transitioning from one page to another.  For example when I click on a link (no link in particular); the page calls for the new URL and then just gets stuck.  I can see the address at the bottom of the IE window that is to be loaded but, then I get no progress on the progress bar, and I can then no longer click on any other links.  I can't press the STOP button, I can't refresh the page, the only thing that I can do is Ctrl+Alt+Delete and use the task manager to End Task on the running istance of IE.  This is driving me nuts...anyone have any suggestions where to look?

This is what I do know about the problem:
1.  The problem is intermitant and does not occur on just one page - or - does not occur with just one link (it appears to be random).
2.  I have checked the php server and the .ini file and there have been no changes to this file over the past few months during the site development.
3.  I have not modified the code used on my site (heavily) with exception of modifying the CSS file.
4.  To ensure that I did not compromise my own code, I replaced all of my files with backup copies that were taken before this problem arose?

This is what I do know about my code/hardware:
1.  Site code was written in PHP (php version 5.1.2).
2.  Site accesses a MySQL DB (mysql db version 4.1.7).
3.  Site uses various javascripts to handle specific functions.

Please let me know if you have any questions...I'm sure you will.  I'm open to trying anything cause at this point all of my hardwork is useless as the site keeps getting hung-up.

Cheers!