help "bootmgr is missing" no operating system to choose from

If you have a meter, check the voltage on your power supply leads. Maybe one of them has an internal break in the conductor causing intermittent interruption to the supply.
Do you have a spare data cable you can try in case that's got a problem.
Check the mobo where the data cable connects to see if one of the pins has a dry joint. (use a magnifier and examine the solder joints behind the connector for any movement when you wiggle it, or visible hairline cracks in the solder blobs)