Resolvers changing/reverting?

Can someone tell me why my resolvers periodically revert to 8.8.8.8 and 8.8.4.4? I can go to Resolver Configuration, customize the three values, and come back [some time later] and they'll be back to Google's public DNS. It may be after a reboot -- I haven't tested that and I don't often reboot. I just want my resolvers to stay what I've set them as.