boggiling minds everywhere

[66StangGuy]

this is a challenge to all. i receltly noticed my car doing this. i didnt add anything new or change anything recently.

ok heres the problem motor running in park. start to slowly press the gas pedal and at about 1800 rpms or 2000 all of a sudden the tach rpms will drop to 1000.. i continue to press the gas pedal more and it rpms go up..then at 3000 or so it will jump about 800 rpms to almost 4000. if i let go of the pedal then the rpms start to drop and when it gets to 2000 the tack bounces back up a little and then falls back to idle. my conclution was a vaccume leak somewhere.. but i alredy tried carb cleaner and didnt fine any leaks anywhere. i guess my question is what opens or changes around 2000 rpm that is leaking? i though it was the pcv valve but i replaced the hose from the carb..the valve is fine. does the valve have to have a air tight seal to the valve cover? what else could it be? im shure the carb is fine its pretty new. any ideas at all? gas milage and performance suck when this happens. thanks