[Wed Jan 10 21:49:16 2007] [error] [client] File does not exist: /home/robkam2/public_html/403.shtml
eh that just means that your custom error file doesn't exist; which is a seperate issue in its own.
Try searching for 509 or 404 errors that occour about the time that you recieve said error.
As for the MySQL errors, those are saying that your site is reaching the max MySQL connections; you should make sure that you arent using persistant connections (as theyre dirty and dont like to close themselves out too much). If you're still finding you're over on these you should contact your host about it (basically when it fills no one can reach your forum until some connections die out)