MBP (Late 2013 Retina) Randomly shuts down, if no thunderbolt

My Late 2013 MBP Retina has started to randomly Sleep then Shutdown. Happens while plugged in and on battery. Have tried PRAM Reset etc. and no improvement. Purely by accident, I noticed that when my Thunderbolt External Drives are connected, the issue doesnt happen. As soon as I remove them, the device shuts down within 5-10 minutes.