Why does my VR sequence suffers big frame drop every minute?

Hi, I’m finally posting this question cause i can’t find any answers anywhere.

I’m working on a cinematic VR exprerience for Oculus, using UE4.16 and the sequencer.
The cinematic is about 3 minutes 45s long
I’ve done several versions, beginning in UE4.15 then switched to 4016. The scene is very simple and I’m experiencing the same problem since the beginning that I can’t get rid of : huge fps drops occurs PRECISELY every minute of the sequence.
So one drop 1mn after beginning, then another one 1mn later and then another one again 1mn later.

Between those drops, everything works perfectly. I’ve optimized the graphics to the max, no dynamic lights, forward render etc…

It seems the problem is elsewhere, due to the fact that these big drops are occuring at precisely one minute interval.
Needless to say that nothing particular happens at these moments (draw calls, new level…)

I’m kinda going crazy over this issue that’s been following me for months now.

Can anyone help me pleaaaaaaaase?