Most likely there is a problem with session management on the server.
use the session test page in the following thread to make sure the server can properly save session data:
if the session test fails, you will need to contact the host to have them correct the problem, most likely a bad session.save_path setting in the php.ini file.
for the css issue, what css are you loosing?
do you have an example i can see before and after?
without knowing specifics on what styles are being effected, i cant give specifics on how to prevent it.