I'm sure its the .htaccess file as the problem doesn't go away with the admintools plugin turned off, but it does if I replace the .htaccess file with the one shipped with Joomla.
I've put in all the obvious exceptions noted in other threads
components/com_dtregister/captchasecurityimages.php
components/com_dtregister/success.php
All the .js files listed in the source code of my page are accessible.
Stranger still is that if I reload a page that has the problem on it, the problem goes away, the calculations are performed correctly and everything works.... so the problem is ONLY on first load (but I can't make end users reload).
I'm wondering where to go now to debug the problem, as I'm stumped. Any suggestions would be appreciated.
Thanks