It's ok to use new drivers but, it's possible you had wrong tweaks/settings coming from the old drivers, that would cause issues to FSX with the new one.
- Try to use the customized driver setup option, and make a "clean" install.
- Reset the FSX settings to default, by removing the following file:
%APPDATA%\Microsof\FSX\FSX.CFG
This will force FSX to rebuild a new config with all default settings. After doing this, be sure you apply the HIGHMEMFIX tweak in the Addon Manager, save the settings and restart FSX.