Then you need this in your fsx.cfg file inside the GRAPHICS section.
[GRAPHICS]
HIGHMEMFIX=1
SHADER_CACHE_VERSION=1 // (this is to force a shader re-compile) as long as this number doesn't change, it won't be re-compiled again.
Umberto, see g2d.dll in the main FSX folder and look around the variables that surround the string HIGHMEMFIX, you'll be surprised