It seems not to be CB as sites without CB have the same problem.
It's either server-related or another component-related.
Could you please:
1) tell exactly server software type and versions (e.g. apache, phpinfo results, etc)
and which joomla/mambo version is installed ?
2) which extensions are installed ?
3) if you install just joomla on same server does it also happen ?
4) if you install just joomla and CB ?
Thanks for helping us finding that problem (we heard reports on joomla forum on joomla 1.5.6 for this problem).
Please reply with points 1 and 2 already...then with 3 and 4.