[ubuntu] xenial 64 bit often almost boots but goes to a black screen

Often when booting (maybe 20% or 25% of the time) the process goes almost all the way but before showing the login box it seems to get stuck and shows only a black screen. Unfortunately the only solution in those cases is to hold the power button to shut down, then try again. I'd rather not be doing that kind of shut down. And anyway it's a problem.

I've been searching and found a tip that suggested putting a delay in /etc/init/lightdm.conf. I added a delay of 3 seconds and that seemed to fix it for a few days, but now the problem is back.

Any advice would be most appreciated. Including advice on even where to look. Thanks much in advance!!

Ubuntu Studio 16.4 xenial 64 bit. The computer is a bit old: a sony vaio from 2010. Also I have it set up to dual boot with windows 7 though I never use windows (why would I? )