Autopilot locking/freezing

[Dreamflight767 4]

Hi:

 

I've been having an issue with the CRJ where/with the autopilot basically freezing and does not follow a route.

 

I left KGRR this morning and flew runway heading from RY35.  I turned left and went direct to ELX.  Turned the autopilot on, hit NAV on the MCP but the autopilot flew in NAV mode disregarding the course to ELX. I tried putting the autopilot back into HDG to go direct ELX again but the autopilot won't respond.  I had to move my joystick to get the autopilot to disarm so I could turn the airplane back on course.

 

On a previous flight, the autopilot was following course but blew right past a turn to the next fix.  It just kept flying straight.  Again, had to move my joystick to basically "wake" the airplane/autopilot up.

 

I'm not sure what's causing this.