I submitted a ticket to PMDG and what they told me was this aircraft uses a software called SIMCONNECT and because it does you might have problems changing from a aircraft in your fleet to the PMDG737NGX 800/900
the notes below are copied from the Documentation folder
KNOWN FSX’isms TO AVOID
The best way to think of the PMDG 737NGX is to approach it in terms of a stand-alone simulation that is using your FSX installation in order to provide a world in which to simulate the 737’s flight operation. We have put a tremendous amount of time and energy into make certain that this experience is as seamless as possible, but occasionally we find unintended consequences of our efforts to provide you with the most accurate simulation possible. A few notes to keep you from running into trouble
There is a common misconception that when loading a complex simulation like the PMDG 737NGX that you should first load the default Cessna into the simulator, then load the PMDG 737NGX. This is not a factual conception and in fact we do not recommend the practice at all. What is required however is for the default Ultralight at Friday Harbor to be your default startup state in FSX.
• One shortcoming of all PMDG products for FSX is that our products do not like to be “re-loaded” on top of themselves. This means that if you have one livery or variant of the airplane selected and running within the simulation world (say, a 737-800 with winglets) and you wish to change liveries to a different 737800 with winglets, you will need to shut down the simulator and then select the new variant or livery you wish to switch to from scratch. If you load an NGX livery or variant on top of itself you will see strange issues in the cockpit, FSX may CTD or otherwise crash etc.
• Setting the PMDG 737NGX as your default flight can cause the same behavior described above from reloading it on top of itself. The default flight is loaded in the background as you start FSX and thus if the NGX is your default flight and you then select another variant, it loads on top of itself even though you may not actually see it happen. Keep the default flight as the ultralight at Friday Harbor and you won’t have any issues with this.
Hopefully others can benefit from the above
thanks for the replies
So in reality there is nothing wrong with the aircraft