UE 5.6 - unwanted flickering and "mirage" effect

I create a simple PCVR scene with UE 5.6 for Meta Quest 3, using the VR Template.
I use deferred rendering with DX12 - but also tested with DX 11, same issue.

The image doesn't seem to be stable. Everything has an unwanted "mirage" effect. Which looks like a "heat effect" in summer, where you see edges bending or weirdly flickering.

I can't find any solution to this problem.

Thx for any help on this,
appreciate it!