@Bobsie: Have you tried checking your TV settings to see if there are any options that might automatically adjust the volume under certain conditions, such as the content you're watching, or the ambient sound in the room. If you have a Samsung TV, try heading to Settings > General > Intelligent Mode Settings > Adaptive Sound+ > Off > Adaptive Volume > Off. Furthermore, head to Settings > Sound > Expert Settings > Auto Volume > Off.
Having the exact same with the Q990B... Don't have a Samsung TV, the soundbar is connected to the TV via Optical (happens when it's connected with HDMI as well)... Hard resetting is not working by pressing the minus and plus on the soundbar ... Goes to volume 8 as well, updated the firmware to the latest version available
Turns out the soundbar had gone into shopmode ... to get it back out turn off the soundbar, than press the volume down button on the soundbar. (The default option was to turn it off so no fiddling required) ... Documentation could be added in the user manual on these kind of issues.