Been running WordPress on PHP 5.3.5 for some time now but recently the system has developed a terminal problem.  Every few loads I'll get a 500 error - maybe 6 to 10 page refreshes.  There's no logging, no problems, and the page returns to normal on the next refresh.  Both my load balanced servers have the same problem on 2 WordPress separate sites each.

Machine is Windows 2008R2 (so IIS7.5).

I have tried for 10 hours to find the problem with no success.

Failed Request Tracing identifies: FastCgiModule / ExecuteRequestHandler / PHP53_via_FastCGI
The error code is: 0x00000000  (helpful)

I switched from using an Application Pool Identity to a named account, but no joy.

Occasionally I correlate the crash with the spawning of a new php-cgi.exe process, but not always. To make it crash I just sit on the home page and hit 'refresh' a few times - normally takes less than a minute to crash it.

When it works, it's very fast. Then every so often it'll just hang for about 20 seconds, then go bang.  Then happily continue on its way.  Nothing in "php-errors", nothing in the Event Log, nothing. 

I can't recall any server changes and I'm hesitant to blame code since 4/5 page loads work properly.  I tried installing a newer version of PHP (5.3.10) but that had no effect so, in the interests of science I rolled it back.  I have two effectively identical servers and I've only been playing with one so I have, in essence, a control group - not that it's helping.

I've read 1,000,000 posts on this topic and fettled 128.4 PHP.INI settings to no avail...

Help.