[imp] strange apache2/php problems

[Sean D.]

Hi all.  I apologize in advance for a somewhat "off topic" post, because I 
suspect my problem lies in apache/php settings somewhere but as it seems to only 
manifest itself in my webmail, I turn to you guys for help.

The Issue:
Every couple of days or so my webmail seems to freak out and stop responding. 
the corresponding apache threads hang indefinitely in either "sending reply" 
state or "gracefully finishing" (according to extended-status page of apache2). 
  What the user experiences is the ability to login but not much else, clicking 
a link (be it the Inbox link, or clicking on a message, or anything really) the 
browser just hangs and then eventually after a long time, it times out.  Each 
time they try clicking again (the same or a different link) after this starts 
occuring, a new apache thread spawns and also hangs. This creates a lot of 
threads that do not die until a restart of apache and then the problem is solved 
for another day or two.  Strangely enough, while this is occuring, another user 
coming from a different IP might be able to login, check mail normally, and 
logout but the users that are experiencing the problem can kill their session 
(close all browser windows) and come back again and still it locks up on them. 
It is not browser specific (i've tried IE6 and firefox) and it even is 
reproducable on either the Alpha ver. or the latest stable versions.

Yes, I know this doesnt sound like an issue with horde so much as an issue with 
apache or php (and I agree with that assessment) but i have lots of php/mysql 
sites on this server and none seem to have this problem except for horde.

The server is:
RHEL3 Taroon 3 (pretty much stock RHEL3 packages)
PHP 4.3.9 (was RHEL3's stock 4.3.2 until recently i upgraded to try and fix this 
issue)
Apache 2.0.46
MySQL 3.23.58
Courier IMAP
Qmail 1.03
ClamAV 0.80 (using qscanq)
Spamassassin
(btw, php's memory_limit setting is set to 64M)

Horde uses IMP for auth which uses IMAP. I use the vdomain hook in both the 
Alpha ver. of my webmail and the stable to handle multiple virtual domains.  the 
backend stuff is all mysql, no custom session handlers or anything unusual. 
Alpha and stable use separate databases.

I can provide additional info if anyone is interested. (i.e. phpinfo(), horde 
test.php output, pastes of apache2's extended-status output while its got all 
the stuck threads, etc.)

Even guesses or nudges in the right direction would be greatly appreciated at 
this point.

Thanks for reading all this,
Sean

p.s. almost forgot to mention, both apache and php error logs show no errors and 
'top' shows cpu usage as mostly idle.  This server is not high traffic and 
generally not under much strain at all.