I do delete the FSX.cfg file.
Like any other file.
I restart FSX which regenerates this file for me.
That's exactly what is supposed to do.
I add HIGHMEMFIX=1 at the end of [GRAPHICS], I save but this one disappears??
Are you doing it in the opposite order maybe ? As I've said:
You must first remove the FSX.CFG and then, after the sim has regenerated one with all default values, add the HIGHMEMFIX=1 tweak to it.
I must not put the tweak in the right place?
It's in the correct place but, you must be sure you add it *AFTER* the sim has generated a new one, and be careful not corrupt the file, for example using the wrong editor (use Notepad), or making any other kind of mistake like adding extra commands, spaces, wrong letter, editing the file while the sim is open, etc.
Basically, any mistake you do, the sim will check the file and remove it AGAIN, because you made it invalid.