Hi, I'm having some issue with my fsx, I was flying from Glasgow to Seattle with PMDG 747 and after 2 hours flying,crusing at FL380, the autopilot went crazy: LNAV VNAV CMD were set, but the aicraft suddenly started to bank to the left constantly (F/D was inviting me to turn to the right following the FMC) and the altitude started to fluctuate between FL380 and FL 370. I tried to disconnect the autopilot, but It didn't let me take control of the aicraft, so I had to end the flight....
I also use active sky 2016 while flying.
please help me, I don't know what is it!