I've heard the same thing - that DK's lifesteal sword is what causes the heartbeat problems. As far as I can tell, both Chaotic-Lords and Necromancers have blade spells that lifesteal and don't cause heartbeat errors. Is there a reason that they couldn't just have the spell's version copied to their swords with a check for the death knight pet wielding it to actually activate?

It's sort of the middle ground between removing the sword and keeping their functionality, I think. Of course, the information I have could be bad, or it could be the check for the wielder itself, or any number of unforseen difficulties could abound!
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