Ok, I just purchased the 707 v1.3 and took off happily with minimal problems. I managed to get the INS programmed without issue as I'm pretty familiar with it having used it in the 727 quite a bit. I was able to engage the autopilot in heading mode and controlled my climb with the pitch wheel, still no issues. I engaged nav mode and began tracking my INS course just fine. Then at about FL280 or so the autopilot clicked off for no reason that I can surmise. Everything was fine up to that point. Now I'm mid flight having tried everything I can think of, but the auto pilot will not re-engage. I click the autopilot button, it makes a noise, and does not engage. What would cause it to click off after having been engaged for 3/4 of the climb and not re-engage? 2100nm is a long flight with no auto pilot...

Thanks in advance for any help.