Major Audio sync problem with ATEM TV HD + WP + OBS+ Youtube

We recorded a live climbing competition semi-finals yesterday with this setup :

ATEM TV studio HD
6 x cameras ( 3x SDI and 3x HDMI)
1 Laptop sharing its screen (HDMI)

Makie Mixer with both broadcasters and an ambiant mic going into the atem (XLR right and left)

THE ATEM program out (SDI) going to the WebPresenter (SDI IN)

Webpresenter going to the laptop running OBS studio (same laptop that is sharing it's secondary screen to the atem)

OBS studio broadcasting to Youtube live

Everything is set to 720P

My problem is this. Every so often (randomly it seems) the audio just starts lagging big time , like 5 seconds after video or more. The solution we have found for the moment is to stop and restart the OBS stream. Youtube doesn't seem notice the momentary offline and continues streaming, its a little hick-up on the broadcast.

Here is one example :

Notice the sound is in sync with the blond girl, then we switch cameras for a brief moment for the other guess to take a seat and when we come back the audio is completely off. Notice the audio going completely silent for a second when the camera comes back to the guest...

Any toughs ? What could be causing this ? There are lots of variables , and we are quite puzzled ...

Any help would be really appreciated

Thanks in advance !

-L