Altitude Hold Issues Post Re-Install

I've just completed a complete re-install of both FSX and PDMG. I've been flying and enjoying my 747-400X for almost 3 years now but have run into an issue with altitude. Both climbing and holding the desired altitude.I have enough knowledge and confidence by now to work the FMC. And although the Flight Plan is in with the speeds and climbs, my bird just does her own thing until she stalls. Even when I use the Vertical Speed + or - it still has no control over the altitude of the Air Craft.Can anyone (Dan Turbine 777 perhaps? ) suggest what I might be missing with regards to my F.P. or instruments? Or... may I have done something during my re-install that is causing this?I've read other threads on this but mine seems to be a little different.Any and all suggestions would be appreciated.Thanks in advance for reading this.