Well, #4 sounds like you just have jugg tanks or something. The other two types won't lose aggro very frequently. Jugg tanks probably need another threat buff. One thing they can do, though is get an on-use power relic and pop it at the beginning of the fight, then on cooldown anytime they're main-tanking (and yes, I know that's not ideal).
Edit: what I'm saying is threat loss isn't really a problematic boss mechanic. It's a problematic class mechanic.
the fact that a year after launch Jugg and guard tanks STILL don't produce eneugh damage to keep threat is dischourageing to say the least