Ah, but testing! Did you go looking for leaks on your intake i.e. Around the gaskets with a spray or stethescope? Did you check all the individual components that have a vacuum line routed to them?
The basic idea is if there is air getting in somewhere, the engine will act just like having your foot lightly on the throttle. Speaking of which, check that nothing is interfering with your throttle plate or cable or gas pedal. Had my floor mats catch me like that once, holding the gas pedal slightly applied.