It's nothing crazy like a leaky hose causing the inlet manifold to vent to atmosphere? This will kill an engine instantly. If the leak is small it could explain the idle problems and lack of power under load. When warming up, if the leak gets bigger it will suddenly just die.
So leaking hoses, leaking inlet manifold etc?
Probably not much help just trying to think of something not yet covered and also doesn't involve parts necessarily at this point.