CRON error messages

[trawets]

Hi all
I run cronjobs on my site and under php 5 and I recieve the following error codes I do not recieve these when running php 4, could someone advise on what these codes mean and how I might correct them
I Thank you in anticipation
<br />
<b>Warning</b>: session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cookie - headers already sent in <b>/home/mysite/public_html/joom15/libraries/joomla/session/session.php</b> on line <b>423</b><br />
<br />
<b>Warning</b>: session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cache limiter - headers already sent (output started at /home/mysite/public_html/joom15/libraries/joomla/session/session.php:423) in <b>/home/mysite/public_html/joom15/libraries/joomla/session/session.php</b> on line <b>423</b><br />

<br />
<b>Warning</b>: ./cache/e15d96b612d132bd0ebc835041a0a222.spc is not writeable in <b>/home/mysite/public_html/joom15/administrator/components/com_feedgator/inc/simplepie/simplepie.inc</b> on line <b>1773</b><br />

[wysiwyg]

It's possible you have output before or after the respective opening and closing php tags in the file that's claiming headers have already been sent.

A byte order mark can also cause this, depending on what program was used to edit the file.

Check session.php for blank lines before <?php and after ?>.