[uf-rest] AHAH Returning Mysterious 404s

[Andy Staines]

Good day
I am hoping someone can advise me
I am the author of a WordPress plugin (php) that makes various uses of  
AHAH (straight from the wiki here). 99%+ of my users have no problem  
but two or three receive 404 OK errors returned from each call.
We have investigated everything as far as I can tell. The environments  
are not 'different' - Apache/Linux php4 or 5. Permissions are not an  
issue.
If I paste the url of the php file that is being sent to the js/AHAH  
routine directly into the browser it returns the correct content with  
no problem. And typically of course, I cannot reproduce the error but  
have seen it on a users site.
My javascript is, shall we say, 'recently acquired' but I am not doing  
anything complex - simply populating a div or two.
Can anyone possibly suggest where I should look for the source of the  
problem. before I tear the rest of my hair out?
Any help would be much appreciated
Thanks
Andy