problems debugging big site with occasional long process

[openandout]

I have a loop somewhere in my code, but I can't find it.  The site has lots and lots of includes and various functionalities which make tracking it hard.  I don't know which file, where in the code, which request is causing it.
The site is http://www.myowndictionary.com and the code base is larger than it appears.

So far I have checked several things:
1) postgresql queery log but when the process is running it is not making a query
2) mysql query log but the queeries have stopped when the process is looping
3) I have added the pid to a log file by using PEAR's Log class like this
    $pid = getmypid() ;
    $Log = &Log::singleton('file', '/var/www/myowndictionary/out.log', $pid );
so it is in front of all my Log entries.
BUT the pid which is the long running process's PID (viewed using 'top') has never matched the logged PID

So, I don't even know which process and which request start the thing.  I can say that it seems to happen on viewing just the home page, normally, and it's never happened to me, but obviously something is going on.

I need an idea what to try next as my options are running out, as is my patience with this untractable bug.

any help anybody can give is welcome, I'm willing to spend lots of time to get this thing discovered.
thank you for your time,

joseph.