Hello All.PDMG 747-400. All is working properly but for some strange reason, the VNAV is not!!Climbing and descending on its on free will.Even when disconnected, I have no control over the altitude.Been flying for 3 years and have Angle of Attack DVD, followed instructions but, still going crazy.Any suggestions as to the reasons and how I might be able to fix?Flying from EDDF-KBOS and am at 41000 (give or take 5000)Thank you ion advance.David Bell

You would be better asking this in the PMDG forums.