Random errors from imap_open using PHP

I have a PHP app which uses imap_open...

$hostname = '{mail.webfaction.com:993/imap/ssl/validate-cert}INBOX';
$inbox = imap_open($hostname,$username,$password);

Most of the time it works perfectly, but there seem to be plenty of failed connections. My error_log gets full of these kinds of errors:

PHP Warning:  imap_open() [<a href='function.imap-open'>function.imap-open</a>]: Couldn't open stream {mail.webfaction.com:993/imap/ssl/validate-cert}INBOX in /home/.... on line 50
PHP Notice:  Unknown: Retrying PLAIN authentication after Authentication failed. (errflg=1) in Unknown on line 0
PHP Notice:  Unknown: Can not authenticate to IMAP server: [CLOSED] IMAP connection broken (authenticate) (errflg=2) in Unknown on line 0

Sometimes the second line says this:

PHP Notice:  Unknown: Can't connect to mail.webfaction.com,993: Connection refused (errflg=1) in Unknown on line 0

I've tried different connection strings, using tls on port 143, or no-authenticate, but same problem.

The errors tend to bunch up e.g. today I had 14 errors between 10:03 and 10:16, then 2 between 11:42 and 11:43, and then 11 between 12:02 and 12:44. The app was being called about once a minute.

I'm guessing that the problem is with mail.webfaction.com being unreliable. Or am I doing something wrong?