Does the PMDG uses the key used by ParkMe (Ctri+Shift+P by default), with its own custom command that starts doing something @ 18000 ft ? Given the use of the P, maybe it has something to do with Pressurization ?
You can change the ParkMe key assignment to another key with the "Change key assignment" option under the "Couatl powered plugins" menu, or you can just remove the FSX\fsdreamteam\Couatl\ParkMe folder, which will disable it entirely, since it's not really needed anymore, because its functions has been integrated into GSX.