I've checked for vacuum leaks and can't see any. I am stumped as to why it's suddenly started cutting again. I don't see how it's possible if the MAP isn't getting the pressure! And it's definitely boost cut because if I drop the boost a few PSI it will boost perfectly. Maybe the MAP is faulty? I'm still going to put my spare on there and see what that does.
But anyway, I bit the bullet and bought a HKS fuel cut defencer off eBay for $60. Hopefully that fixes it, or at least it'll tell me the problem is somewhere else.