Try turning the belt around and running single loop first. We went through a similar problem about a year ago and it drove us nuts. It is still the one thing that we have never figured out why it changed, but it did.
When you have to run belts upside down, which is a semi common problem I have seen over the years building guns you check the arm on the belt feed pawl. Sometimes these can be bent forward a little too much. When not properly aligned they will rest on top of the links instead of falling behind the link and resting on top of the cartridge. The feed pawl must seat properly in order to push the linked ammo into the gun.
Either way the gun isn't advancing the belt correctly by what I gather. Its firing the first and second directly about the trunion. If you watch carefully I belt the belt is back feeding, if so its an easy fix.
Check the belt holding pawl spring tension. I have seen that the springs in many of the Izzy .308's are too stiff and cause feeding issues. If all of the above have yet to fix your feeding problems you can remove the holding pawl and remove the spring and then try shortening it by one or two coils. I don't know if I'd recommend removing more than that.