I had this problem recently. Lost about 7 caps. I tried the solid caps and the ones with a removable rubber plug. Lost both styles so I don't think it was a venting issue. The last time I installed them I added a super thin layer of rtv gasket maker to the edge of the cap before pounding the...
It sounds like it is probably something with the feed speed board or something but the other thing to check might be the ground clamp cable. Just make sure it doesn't have a corroded connection anywhere and it has a clean connection to what you are welding.
I had about 15 baby Gardener snakes hanging from my pasture fence within an 80 yard stretch. I saw your post and I believe it is the work of the shrike. Pretty interesting!