I did now a lot more testing and i can say that the sim does NOT crash with a 30 FPS Cap and active DX11. My GPU ( RTX 2080 Super ) is under these settings most time at round about 60% under load.
(EDMM Scenery, FBW A320, Settings at High, 3440x1440 resolution)
I did a long haul flight with the A330-900 (standalone mod) over 9 hours and no problemes there.
I did also a lot of A320 (FBW Mod) short term flights and had no issues.
I also let the sim 8 hours in the menues and nothing happend there.
I tryed other DX12 games and they run without problems. I also did a DX11 test on WOW (world of warcraft) and nothing happend. all that tests without FPS limit and 100% gpu load.
So i can say my hardware is OK. but with some reason MSFS dont like full power DX11. (in my case)
maybe i will set higher FPS limits with the nvidia tool to see when the critical point is reached.
Thanks for that very nice tip MichaMMA 
(I editet the topic of the thread)