I’ve started getting same last few weeks only. Perfectly smooth on ground and climb then spikes every few seconds in cruise which is normally when I get best smooth performance. It’s intermittent (some flights no issue) but more frequent lately. Very annoying.
I have checked Task Mgr and it’s MSFS itself, closing down my add ons like FSUIPC, RealTurb and Coatl/GSX does nothing. Even turning off clouds does nothing and lowering TLOD just hides the spikes (yellow instead of red).
5800X3D 4900