IMAP SSL Problem - Must restart courier-imaps service several times

[GPDCenter]

Hi all,

we have an issue with Plesk 11.5 and IMAP with SSL.

Our configuration is:
Plesk 11.5.30
OS: CentOS V6 r2.6.18-028stab101.1
Postfix Mailserver

SSL wildcard certificate from AlphaSSL.

Many times a day we are not able to connect via IMAP and Port 993 to the Mailserver (SSL/TLS is activated).
POP is working fine. Once i restart the courier-imaps service the connection works fine several hours.
Connection on Port 143 (IMAP without SSL/TLS) work without any issue.

I figured out, that i run in this "hang up" if i try to connect via Mac mail.app and IMAP (btw mail.app don't work
with IMAP accounts correctly, after update to 10.9.2 also not).

Are there any known issues with restarting courier-imaps to get propper IMAP connections?
Any suggestions?

Thank you in advise
Goran