lock up very hard to get restarted

Mac mini mid 2011 running MacOS Sierra

Had this problem a couple weeks ago and seemed to get corrected. Yesterday logged off and returning logged back on and computer started the beachball churning. and all of sudden the screen locked up and had do a power off reboot.

The only common thing is that I had an auto lock and password set on sleep.

Took this off. But when I logged off it auto locks and requires a password to unlock. This when I had the problem logging back in.

Would not reboot. Got gray screen with boot bar taking for ever to get to 60% and then stopping forever.

Did the recovery and ran Disk Utility First-Aid, did the PRAM reset, nothing!

Unplugged everything and attached original keyboard and mouse, again nothing!

Then rebooted and all of sudden computer restarted.

Something is causing this, not sure where to look... USB cables, HDMI cables?