No bootable device found, press f1 and boots just fine

I have an inspiron 13 7000.  after windows 10 upgrade it became painfully slow.  i did a clean install of windows 10 form USB.  Now if i don't use the laptop for a prolonged time (4+ hours) i get  "no bootable device found pres f1 ...." message.  If i press F1 it boots up just fine.  There are no other issues.  I've flashed the most resent bios.  i suspect this  has something to do with me deleting all the partitions when i reinstalled windows (if it does, then shame on dell).  any idea on how to fix it?