DNS unstable

Hi all,

I don't know what the reason is, but usually I have problems with my DNS.
When I boot my Windows client I have to attempt several times to connect to my Flexshares.
Sometimes I lose connection to my web configuration and I have to switch from "Obtain DNS automatically" to "<local ip COS box>"
My COS box 7.9.1 is running in gateway mode.
Please tell me what you need to know to solve this, but don't make it to complicated please ...

John