postfix can't connect to mysql

[phh1]

Hi, sorry for my english.
time to time my server have problem with connet postfix to mysql and pure-ftpd-mysql to mysql only after restart server it's work again and alway after 5-10 hour error repeat.
Postfix log:
Feb 23 13:32:21 hosting postfix/smtp[12260]: fatal: mysql:/etc/postfix/mysql-virtual_relaydomains.cf(0,lock|fold_fix): table lookup problem
Feb 23 13:32:22 hosting postfix/error[12262]: fatal: mysql:/etc/postfix/mysql-virtual_relaydomains.cf(0,lock|fold_fix): table lookup problem
Feb 23 13:32:23 hosting postfix/qmgr[12259]: fatal: mysql:/etc/postfix/mysql-virtual_relaydomains.cf(0,lock|fold_fix): table lookup problem