high sierra start volume lost

I have a brand new iMac with a 3 TB Fusion Drive. Since I installed the High Sierra Version with The new file format, my system runs well until it goes into sleep mode and after a few hours I have just a blinking icon - symbol for a directory- on the screen and you can only restart thru a hardware reset. The icon shown indicates that the system is not recognizing the start volume. Anyone has the same problem or any idea🙏 It is a shame that Apple only came up with the advice not to install High Sierra , after they released the first Beta. 😡

I already brought the Device to an Apple Store and they just loaded the latest Sierra version on the computer. I than updated to the official High Sierra Version and retired by data out of the Time. Capsule backup. Guess what - same issue all over again 😢