Autopilot

I very recently bought/downloaded the E170/190 combo pack, and have flown twice now, both times with the 170. The acft flies great with autopilot on......at first. HDG, A/T, and ALT engaged for 2/3 of flight {no FMS FLT PLAN loaded}. At a random time in the flight, the auto pilot goes crazy. Then its as if nothing is engaged, but the Pilots PFD shows these items engaged. The acft starts flying in a circle, losing altitude and A/T unresponsive.

I try to disengage the buttons for each of these, but it has no effect. I have to turn off the main autopilot and fly manually. HELP