Email SMTP stops working after 1-2 days

[Flash951]

Email SMTP stops working after 1-2 days, without any changes in configuration. 
When replace email address all places, it will work again for 1-2 days, then the same thing over again.

1. Advanced Parameters/E-mail/Test your email configuration: "Error: Please check your configuration"
2. When replacing email address with same settings, get: "Failed to authenticate on SMTP server with username “x" using 2 possible authenticators."

Strange thing, if I fool around and replace with another e-mail address all places like: 
Advanced Parameters/E-mail/Email
Preferences/Store contacts/Contact detail 
Customers/Customer Service/Customer service options

Then it will finally work again, but only for 1-2 days. Then the same thing will happen over again:
Advanced Parameters/E-mail/Test your email configuration: "Error: Please check your configuration"

Server:
PrestaShop 1.6.1.5
Apache/2.2.15 (Red Hat)
PHP-versjon 5.3.3
 

Someone got a clue, please help!