Judah is a mental midget in the ring. I doubt that has much to do with stamina, or lack of it. Judah is what he is. The guy physically had all the tools, he just never had what it took to be an ATG where it counts, in his heart and his head. People need to stop hyping this guy. No trainer, no-one, could've changed his destiny.
If Zab only had the stamina, chin, athleticism and will as Pacquiao he'd be really great. And if Wlad Klitschko only had the chin of George Chuvalo he'd be really really great. And if Dwight Muhammad Qawi only had the height of Bob Foster he'd have been really, really, really great.
He definitely has the athleticism of Pacquiao. He's probably one of the most naturally gifted athletes in the sport of this era. He's got natural hand-speed, power, reflexes, movement etc. There's really not many fighters with Zab Judah's skill set. His issue is a mental one. I've no idea what goes on in his head during fights but that's the main reason why he's never fulfillled his potential.