You didn’t posted what memory model/brand you have, but since you have 5900x Ryzen, x570 mobo and 3090 it sounds unlikely that your memory works just in 2400 MHz - that’s why I asked for checking XMP in bios.
Did you selected parts for your PC yourself, is RAM from your old PC?
Also big question is how are your resolution settings in steam, in MSFS and so on. Many factors can effect performance and introduce stuttering. Also - do you use any mods? If so, remove them and trhy again.