the video is good. A couple of things:

1) it does not look like you have enough tension on the forestay and the jib luff. Rigging tension looks soft but this could be because you are not sheeting hard enough on the main sail.

2) you are not sheeting hard enough on the main sail - not close...not by a kilometer. I normally control the mainsheet while driving and with a 10:1 sheet purchase in that kind of breeze, I have two wraps on my hand as I bend my legs and I put the finishing touch on the mainsheet tension by extending my legs with everything I have. crank it. The leech of the mainsail should be very tight and you control the twist with the downhaul.

What is happening with the loose mainsheet is that as the boat pumps the rig as you go over waves, different areas of the sail are picking up slight shifts in the wind as the sail "tomahawks". With that much twist, the top of the main usually starts to get more power as the boat slows down into the next wave - this causes the driving force to climb higher up the mast which turns into more pressure on the bows and drives them deeper. The F18 has a lot of sail up high - you need to contain it in breeze and waves.